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.

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

java/google/registry/tools/resources/README.md

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+
+# Adding Client Secrets
+
+To use the nomulus tool to administer a nomulus instance, you will need to
+obtain OAuth client ids for each of your environment.  There's no reason you
+can't use the same client id for all of your environments.
+
+To obtain a client id, go to your project's ["credentials"
+page](https://console.developers.google.com/apis/credentials) in the Developer's
+Console.  Click "Create credentials" and select "OAuth client Id" from the
+dropdown.  In the create credentials window, select an application type of
+"Other."
+
+When you return to the main credentials page, click the download icon to the
+right of the client id that you just created.  This will download a json file
+that you should copy to this directory for all of the environments that you
+want to use.  Don't copy over the "UNITTEST" secret, otherwise your unit tests
+will break.