In the WFE, store the User-Agent in a `context.Context` object. In our
gRPC interceptors, pass that field in a Metadata header, and re-add it
to `Context` on the server side.
Add a test in the gRPC interceptors that User-Agent is properly
propagated.
Note: this adds a new `setup()` function for the gRPC tests that is
currently only used by the new test. I'll upload another PR shortly that
expands the use of that function to more tests.
Fixes https://github.com/letsencrypt/boulder/issues/7792
04dec59c67