This is a change in direction from the approach so far:
- OAuthClient.refreshAccessToken was very nearly getRequestMetadata as has been
implemented in some other google-auth-library languages.
- rather than modifying getAccessToken and refreshAccessToken and using
getAccessToken as main 'interface' method for the credentials client classes,
it's better to introduce getRequestMetadata that's a slight modification to
refreshAccessToken and use that instead.
- refreshAccessToken is not removed for backwards compatibility, but is
deprecated.
- the URI is the one being authorized
- this is a breaking surface API change
- in this commit, the affected classes do nothing with the URI except pass it
through
