add bigtable support for per-operation options

[devbww]

Add an Options options = {} argument to the constructor of the client class(es). Merge these options with the default options for the service, and store them as a member of the client class.

Add an Options options = {} argument to each operation within each client. These options should then be merged with the client options from above, and installed as the prevailing options for the duration of the operation by instantiating an internal::OptionsSpan.

You could then use internal::CurrentOptions() to obtain (a const& to) the prevailing options from anywhere you might need them. Any cleanup for call paths where Options have been passed explicitly is discretionary.

Similar support for the generated client classes was added in #7683, so you might be able to use that as an example.

[devbww]

Note: The options stored in the client class should now include the connection->options(). So something like ...

ServiceClient::ServiceClient(std::shared_ptr<ServiceConnection> connection, Options opts = {})
    : connection_(std::move(connection)),
      opts_(internal::MergeOptions(std::move(opts), 
                                   service_internal::DefaultOptions(connection_->options()))) {
  ...
}

T ServiceClient::Operation(..., Options opts = {}) {
  internal::OptionsSpan span(internal::MergeOptions(std::move(opts), opts_));
  ...
}

[devbww]

Addendum: Please also ensure that the OptionsSpan instantiated during ServiceClient::Operation() is saved and then restored over any work done on behalf of the operation but after Operation() returns. Typically this means for async, paginated, or streaming RPCs. (These cases may already be handled via changes in the common library.)