Support API Key Authentication

[dbolduc]

API Keys are a way to authenticate without needing a principal. This is convenient for first time users, who often get tripped up by Google's crazy authentication schemes.

https://cloud.google.com/docs/authentication/api-keys-use

• [x] Introduce ApiKey support
  • [x] Add an ApiKeyOption
  • [x] Set x-goog-api-key headers over gRPC
  • [x] Update guac.dox to mention API Key authentication
• [x] Set x-goog-api-key over REST.
  • We cannot have an integration test. To my knowledge, none of the REST based services currently support API keys.
  • [x] Make sure we censor API keys in any logs/telemetry we produce. https://github.com/googleapis/google-cloud-cpp/blob/2cb2c7439bb0ffb8984eb730969ff8a03004a295/google/cloud/internal/curl_wrappers.cc#L190
• [x] Add a sample like this in Python.
  • Looks like it'd be nice to have one that creates an API key using our apikeys client too.
  • We will have to clean up any leaked keys too. these are dangerous.

[dbolduc]

This is done.