add app_authentication example

Author: Jeremy Udit

examples/app_authentication/README.md

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+# App Installation Example
+
+This example demonstrates authenticating using a GitHub App.
+
+The example will create a repository in the specified organization.
+
+You may use variables passed via command line:
+
+```console
+export GITHUB_OWNER=
+export GITHUB_APP_ID=
+export GITHUB_APP_INSTALLATION_ID=
+export GITHUB_APP_PEM_FILE=
+```
+
+```console
+terraform apply -var "organization=${GITHUB_ORG}"
+```

examples/app_authentication/main.tf

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+resource "github_repository" "github_repository" {
+  name        = "github_app_example"
+  description = "A repository created using GitHub App authentication"
+}

examples/app_authentication/outputs.tf

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+provider "github" {
+  owner = var.owner
+  app_auth {
+    // Empty block to allow the provider configurations to be specified through
+    // environment variables.
+    // See: https://github.com/hashicorp/terraform-plugin-sdk/issues/142
+  }
+}
+
+terraform {
+  required_providers {
+    github = {
+      source  = "integrations/github"
+    }
+  }
+}

examples/app_authentication/providers.tf

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+variable "owner" {
+  description = "GitHub owner used to configure the provider"
+  type        = string
+}