Add Client-side OAuth2 to HTTP connections

Implement client-side OAuth in non-local HTTP connections.  Also add tests to
verify that the different modes of connection are set up correctly.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=147636222

java/google/registry/tools/resources/client_secret.json

@@ -0,0 +1,11 @@
+{
+  "installed": {
+    "client_id":"SEE-README.md-IN_THIS_DIRECTORY.apps.googleusercontent.com",
+    "project_id":"your-registry-server",
+    "auth_uri":"https://accounts.google.com/o/oauth2/auth",
+    "token_uri":"https://accounts.google.com/o/oauth2/token",
+    "auth_provider_x509_cert_url":"https://www.googleapis.com/oauth2/v1/certs",
+    "client_secret":"YOUR-CLIENT-SECRET",
+    "redirect_uris":["urn:ietf:wg:oauth:2.0:oob","http://localhost"]
+  }
+}