Add static SDK CI workflow which runs on commits to PRs, merges to main
and daily on main.

---------

Co-authored-by: Gus Cairo <gus.cairo@apple.com>
Co-authored-by: George Barnett <gbarnett@apple.com>

This PR follows-up #2206 and fixes the Android build I noticed at
https://swift-everywhere.org.

Hopefully we will soon have CI support for Android
(swiftlang/github-workflows#106) to catch this
sort of breakage sooner.

Motivation:

gRPC checks whether errors thrown from interceptors are `RPCError` and
otherwise treats them as `unknown` (to avoid leaking internal
information). There is a third possibility: the error is explicitly
marked as being convertible to an `RPCError`. This check is currently
missing when thrown from client/server interceptors.

Modifications:

- Catch `RPCErrorConvertible` in the client/server executors when thrown
from interceptors
- Add tests

Result:

Error information isn't dropped