Support CustomWriterIdentity

[johnclara]

Thanks for stopping by to let us know something could be better!

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Is your feature request related to a problem? Please describe.
I think we should be able to set a customWriterIdentity in a CreateSinkRequest. Based on rest api: https://cloud.google.com/logging/docs/reference/v2/rest/v2/organizations.sinks/create

Describe the solution you'd like
Allow setting custom writer identity.

Describe alternatives you've considered

Additional context

[johnclara]

@cindy-peng any update on this? Is this on the roadmap or is there a different way to implement this?