gapic: add top level example(s) of using SetHeaders

[noahdietz]

We have the callctx.SetHeaders package that allows users to set request headers regardless of underlying transport.

We need to add a doc section for setting headers, especially with a system parameter like x-goog-fieldmask.

Note: gRPC requires that the system param keys be lower case, which we should highlight.

Motivation: googleapis/google-cloud-go#9303