[CO-390] Create a 'demo' executable for cardano-sl-wallet-new

This code makes use of the 'Cardano.Wallet.Demo' module and show
how it can be used to spawn a demo cluster of nodes.
More details in the README.md

Author: KtorZ

wallet-new/cardano-sl-wallet-new.cabal

@@ -714,3 +714,21 @@ benchmark cardano-sl-wallet-new-bench
                         BangPatterns
                         TemplateHaskell
                         ScopedTypeVariables
+
+test-suite demo
+  type:               exitcode-stdio-1.0
+
+  default-language:   Haskell2010
+  default-extensions: NoImplicitPrelude
+                      FlexibleContexts
+                      OverloadedStrings
+                      RecordWildCards
+                      ScopedTypeVariables
+
+  hs-source-dirs:     demo
+  main-is:            Main.hs
+
+  build-depends:      base
+                    , async
+                    , cardano-sl-wallet-new
+                    , universum

wallet-new/demo/Main.hs

@@ -0,0 +1,53 @@
+{-| Demo cluster of wallet nodes. See demo/README.md -}
+
+{-# LANGUAGE LambdaCase #-}
+
+module Main where
+
+import           Universum
+
+import           Control.Concurrent.Async (waitAny)
+import           System.IO (BufferMode (..), hSetBuffering, stdout)
+
+import           Cardano.Wallet.Demo (MaxWaitingTime (..), NodeName (..),
+                     NodeType (..), RunningNode (..), startCluster,
+                     waitForNode)
+
+
+-- | Cluster configuration can be tweaked via ENV vars. Each ENV var is prefixed
+-- with the following.
+--
+-- (e.g. `DEMO_NO_CLIENT_AUTH=True`)
+prefix :: String
+prefix = "DEMO_"
+
+
+main :: IO ()
+main = void $ do
+    hSetBuffering stdout NoBuffering -- Instead of LineBuffering
+
+    putTextLn "Cluster is starting..."
+    cluster <- startCluster prefix
+        [ ("core0", NodeCore)
+        , ("core1", NodeCore)
+        , ("core2", NodeCore)
+        , ("core3", NodeCore)
+        , ("relay", NodeRelay)
+        , ("wallet", NodeEdge)
+        ]
+    handles <- forM cluster $ \case
+        RunningCoreNode (NodeName nodeId) handle -> do
+            putTextLn $ "..." <> nodeId <> " skipped!"
+            return handle
+
+        RunningRelayNode (NodeName nodeId) handle -> do
+            putTextLn $ "..." <> nodeId <> " skipped!"
+            return handle
+
+        RunningWalletNode (NodeName nodeId) client handle -> do
+            putText "..." >> waitForNode client (MaxWaitingTime 10)
+            putTextLn $ nodeId <> " OK!"
+            return handle
+    putTextLn "Cluster is ready!"
+
+    waitAny handles

wallet-new/demo/README.md

@@ -0,0 +1,76 @@
+# Demo
+
+## Getting Started
+
+This module provides an executable for starting a demo cluster of wallet nodes.
+It is designed to remove all the overhead of setting up a configuration
+and an environment and to _just work_, out-of-the-box. Minor configuration
+adjustments are however possible via environment variables. 
+
+```
+stack test cardano-sl-wallet-new:demo
+```
+
+This spawns three wallet-nodes running respectively on:
+
+| NodeId | Address        | API Address    | API Doc Address |
+| ---    | ---            | ---            | ---             |
+| node0  | localhost:3000 | localhost:8090 | localhost:8190  |
+| node1  | localhost:3001 | localhost:8091 | localhost:8191  |
+| node2  | localhost:3002 | localhost:8092 | localhost:8192  |
+
+
+## Configuring Nodes
+
+Almost _anything_ from the normal CLI arguments of a node or a wallet node can be
+configured via an ENV variable using an `UPPER_SNAKE_CASE` naming, correctly
+prefixed with `DEMO_` with a few gotchas:
+
+- Flags need an explicit boolean value
+
+- There's no ENV vars mapping to (i.e. you can't configure):
+    - `--topology`
+    - `--tlscert`
+    - `--tlsca`
+    - `--tlskey`
+    - `--logger-config`
+    - `--node-id`
+    - `--db-path`
+    - `--wallet-db-path`
+
+- There's an extra `LOG_SEVERITY` variable that can be set to `Debug`, `Info` 
+  and so forth to ajust logging severity for _all_ nodes.
+
+- When it make senses, variable values are automatically incremented by the
+  node index. For instance, if you provide `LISTEN=127.0.0.1:3000`, then 
+    - node0 will receive "127.0.0.1:3000"
+    - node1 will receive "127.0.0.1:3001"
+    - node2 will receive "127.0.0.1:3002"
+
+  This is the case for:
+    - `--listen`
+    - `--wallet-address`
+    - `--wallet-doc-address`
+
+For instance, one can disable TLS client authentication doing:
+
+```
+DEMO_NO_CLIENT_AUTH=True stack test cardano-sl-wallet-new:demo
+```
+
+
+### Relative FilePath
+
+One can provide relative filepath as values for ENV vars. They are computed from 
+the `wallet-new` folder, so for instance, providing `./state-demo` will point 
+to the directory `$(git rev-parse --show-toplevel)/wallet-new/state-demo`.
+
+
+### State Directory
+
+By default, each node receives a temporary state directory from the system;
+probably somewhere in `/tmp`. This location can always be overriden by 
+providing an extra `DEMO_STATE_DIR` variable with a custom location.
+
+Note that, each default has been choosen in such way that they won't conflict
+if all nodes were to share the same state directory :)