showcase: test with all the RPCs for `client::Echo`

[coryan]

The google.showcase.v1beta1.Echo.Echo RPC uses enums, but the other RPCs work fine. I will be sending a PR for EchoErrorDetails(), FailEchoWithDetails(), PagedExpand(), PagedExpandLegacy(), and PagedExpandMapped().

That will leave unimplemented:

• [x] google.showcase.v1beta1.Echo.Echo because enums
• [ ] google.showcase.v1beta1.Echo.Expand because server-side streaming.
• [ ] google.showcase.v1beta1.Echo.Collect because client-side streaming.
• [ ] google.showcase.v1beta1.Echo.Chat because bi-dir streaming.
• [ ] google.showcase.v1beta1.Echo.Wait because it is redundant, tests for timeouts and LROs.
• [ ] google.showcase.v1beta1.Echo.Block because it is redundant, tests for timeouts and retries.

[coryan]

Blocked on https://github.com/googleapis/gapic-showcase/issues/1592

[coryan]

The enum problem is not something we can solve, we will ignore that test case.