[java-logging] Support CustomWriterIdentity

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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**