github: add support for OAuth with GHAE

Add support for detecting GitHub AE (GHAE) and enabling the OAuth
authentication mode with the GitHub provider.

https://docs.github.com/en/github-ae@latest/admin/overview/about-github-ae

Author: mjcheetham

src/shared/GitHub/GitHubConstants.cs

@@ -32,6 +32,11 @@ public static class GitHubConstants
         // TODO: update this with a real version number once the GCM OAuth application has been deployed to GHE
         public static readonly Version MinimumEnterpriseOAuthVersion = new Version("99.99.99");
 
+        /// <summary>
+        /// The version string returned from the meta API endpoint for GitHub AE instances.
+        /// </summary>
+        public const string GitHubAeVersionString = "GitHub AE";
+
         /// <summary>
         /// Supported authentication modes for GitHub.com.
         /// </summary>

src/shared/GitHub/GitHubHostProvider.cs

@@ -263,8 +263,15 @@ internal async Task<AuthenticationModes> GetSupportedAuthenticationModesAsync(Ur
                 {
                     modes |= AuthenticationModes.Basic;
                 }
-                if (Version.TryParse(metaInfo.InstalledVersion, out var version) && version >= GitHubConstants.MinimumEnterpriseOAuthVersion)
+
+                if (StringComparer.OrdinalIgnoreCase.Equals(metaInfo.InstalledVersion, GitHubConstants.GitHubAeVersionString))
+                {
+                    // Assume all GHAE instances have the GCM OAuth application deployed
+                    modes |= AuthenticationModes.OAuth;
+                }
+                else if (Version.TryParse(metaInfo.InstalledVersion, out var version) && version >= GitHubConstants.MinimumEnterpriseOAuthVersion)
                 {
+                    // Only GHES versions beyond the minimum version have the GCM OAuth application deployed
                     modes |= AuthenticationModes.OAuth;
                 }