view: 🥜 `ViewServer` support for an in-memory gRPC endpoint

[cratelyn]

refs:

• app: 📺 test that Planner can spend notes via mock consensus #4471
• Design a way to use the Rust view server with mock consensus integration tests #3913

mock consensus tests that use a view server must bind an rpc endpoint to a local port. this is unfortunate because it requires acquisition of a global resource, and can over time cause test suites to be flaky / brittle.

this could be abated by adding support for an in-memory gRPC endpoint to the ViewServer and its underlying Worker.

see https://github.com/penumbra-zone/penumbra/tree/kate/view-server-tests-redux.box-grpc-endpoint-experiment as a starting sketch.

see #4471 (comment) for relevant discussion from #4471.

[cratelyn]

i observed the test from #4471 flake in CI, after opening #4585. re-running the job worked, fixing this issue will prevent that flake from occurring going forward.