Add a sciddir_or_pubkey fundamental type

Offers may contain blinded paths to allow for greater recipient privacy.
However, they come at a cost of increased QR code size as the
introduction node requires a 33-byte `point`.

Define a new `sciddir_or_pubkey` fundamental type such that either a point or a
reference to one in a `channel_announcement` can be used. This is
backwards compatible with `point`.

Use this new type for the `blinded_path` subtype's `first_node_id`.

Author: jkczyz

01-messaging.md

@@ -246,6 +246,12 @@ The following convenience types are also defined:
 * `bip340sig`: a 64-byte bitcoin Elliptic Curve Schnorr signature as per [BIP-340](https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki)
 * `point`: a 33-byte Elliptic Curve point (compressed encoding as per [SEC 1 standard](http://www.secg.org/sec1-v2.pdf#subsubsection.2.3.3))
 * `short_channel_id`: an 8 byte value identifying a channel (see [BOLT #7](07-routing-gossip.md#definition-of-short-channel-id))
+* `sciddir_or_pubkey`: either 9 or 33 bytes referencing or identifying a node, respectively
+    * if the first byte is 0 or 1, then an 8-byte `short_channel_id` follows for a total of 9 bytes
+        * 0 for the first byte indicates this refers to `node_id_1` in the `channel_announcement` for `short_channel_id`
+        * 1 for the first byte indicates this refers to `node_id_2` in the `channel_announcement` for `short_channel_id`
+          (see [BOLT #7](07-routing-gossip.md#the-channel_announcement-message)
+    * if the first byte is 2 or 3, then the value is a 33-byte `point`
 * `bigsize`: a variable-length, unsigned integer similar to Bitcoin's CompactSize encoding, but big-endian.  Described in [BigSize](#appendix-a-bigsize-test-vectors).
 * `utf8`: a byte as part of a UTF-8 string.  A writer MUST ensure an array of these is a valid UTF-8 string, a reader MAY reject any messages containing an array of these which is not a valid UTF-8 string.
 

04-onion-routing.md

@@ -1485,7 +1485,7 @@ even, of course!).
 
 1. subtype: `blinded_path`
 2. data:
-   * [`point`:`first_node_id`]
+   * [`sciddir_or_pubkey`:`first_node_id`]
    * [`point`:`blinding`]
    * [`byte`:`num_hops`]
    * [`num_hops*onionmsg_hop`:`path`]