Tracking Issue for authority being Experimental.

In regards to #67.

Specific usages:
- `CallOptions.getAuthority`
- `CallOptions.withAuthority`
- ~`ManagedChannelBuilder.overrideAuthority`~

Start with issue #67 and inspect the listed authority usages in CallOptions.getAuthority and CallOptions.withAuthority; ManagedChannelBuilder.overrideAuthority is marked as already addressed. Determine which remaining usages are covered by the experimental-authority work, and consider the issue complete when the applicable usages are handled consistently.

索引モデルが issue の本文から書いたものです。