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.

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

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+{
+  "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"]
+  }
+}

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

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+{
+  "installed": {
+    "client_id":"UNITTEST-CLIENT-ID",
+    "project_id":"DO NOT CHANGE",
+    "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":"TBj4EcP5c0609ojiy2DIG6wE",
+    "redirect_uris":["urn:ietf:wg:oauth:2.0:oob","http://localhost"]
+  }
+}