Introduce support for pluggable remote helpers (declared via the
optional 'helper' attribute in <manifest-server>) to dynamically resolve
proxy addresses. Route the XML-RPC manifest server connection through
the resolved proxy.
Bug: b/517477903
Change-Id: I3b6b8ea2640bb077521df4b4a9e8a34a8c6ecdad
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/591642
Tested-by: Rahul Yadav <yadavrah@google.com>
Commit-Queue: Rahul Yadav <yadavrah@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>