Introduce auth-provider specific props & use it for social sign in for GitHub (#251649)

ref https://github.com/microsoft/vscode/issues/251648

NOTE: the server side is not quite ready. It doesn't redirect back to VS Code properly, but this should be good to go whenever that is.

extensions/github-authentication/package.json

@@ -18,7 +18,8 @@
     "workspace"
   ],
   "enabledApiProposals": [
-    "authIssuers"
+    "authIssuers",
+    "authProviderSpecific"
   ],
   "activationEvents": [],
   "capabilities": {