Add checks for invocation-id to testbench

[tritone]

@saranshdhingra suggested that it would be helpful to add an option to check correctness of the x-goog-api-client header in the retry test support in the testbench.

This might include the following:

1. Check that gccl-invocation-id is the same for all retries of a given request
2. [if applicable] Check that gccl-attempt-count starts at 1 and gets incremented for each retry.

We could also add a check for x-goog-gcs-idempotency-token when implemented.

This check could be enabled by a data field or query param to the retry_test POST endpoint.