Run Connect conformance tests in CI

[jhump]

We should run Connect's conformance tests against a server that uses Vanguard to do protocol translation. This could help tease out any possible issues that aren't checked in this repo's existing suite of tests (which mostly rely on Connect clients and servers).

Currently, running conformance tests against new server implementations involves adding the implementations to the conformance repo so they are included in that repo's matrix of interop cross-tests. But a "v2" of the conformance tests is under-way, with the express intent of making it easier to run the conformance tests against new implementations. So it makes sense to defer addressing this issue until that v2 is ready to use.

[akshayjshah]

For any readers unfamiliar with Connect's project structure, the conformance tests in question are https://github.com/connectrpc/conformance.