SpannerOptions: document how to pass in client gRPC options like client interceptors

[odeke-em]

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Documentation Request

Kindly please document how to pass in gRPC options and for example interceptors to the Spanner Client as they are very important to inject and assert on expectations like certain headers. I am rolling out x-goog-spanner-request-id support for all the major Cloud Spanner client libraries and for this client I have a PR started #2205 but it is quite a hairy situation to try to add client interceptors so for starters I am using the server interceptor with MockSpanner to verify expectations.

Kindly cc-ing @olavloite @surbhigarg92