Add an rpc whose request message has a repeated field in the method signature

There are currently no methods whose input message type contains a repeated field (with the exception of testing.proto:VerifyTestRequest, which as a repeated bytes field), and, therefore, no methods with a repeated field in their signatures.
This constitutes a test hole, as there are real APIs with these characteristics, and generators need to support this functionality and have tests that verify its integrity.

See also https://github.com/googleapis/gapic-generator-python/issues/413 and https://github.com/googleapis/gapic-generator-python/issues/410 for additional background.