Deprecate storage::ClientOptions

[coryan]

I expect that soon the recommended way to configure a google::cloud::storage::Client object would be to use google::cloud::Options (see #6640).

We cannot atomically deprecate (much less remove) storage::ClientOptions at this point because the quickstart guide still uses it, and we need a new release before we can cleanup the quickstart.

Once that release is out and the quickstart is updated we can deprecate all the functions that use storage::ClientOptions, probably deprecate storage::ClientOptions itself, and schedule its removal for a year (or more if needed) down the line.

[coryan]

Close, this is deprecated already (not removed).