sending an empty rpchttp to worker when proxying

[brettsam]

When http proxying is enabled, there's no need to send the http payload over grpc as well. This PR short-circuits the generation of the grpc payload and returns a static, empty RpcHttp object if we know we're proxying the request.

Note that we still send the entry in the dictionary that maps to the trigger -- but it's effectively a placeholder. This is just in case any worker depends on this entry being present.

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• My changes do not require diagnostic events changes
  • Otherwise: I have added/updated all related diagnostic events and their documentation (Documentation issue linked to PR)
• I have added all required tests (Unit tests, E2E tests)