sync: Add GetApprovedManifestV2 with named request options

Smart sync calls GetApprovedManifest(branch[, target]) over XML-RPC,
which only supports positional arguments.  Adding new request options
that way does not scale.

Add a GetApprovedManifestV2 method that takes a single struct of named
options and returns a struct:

  request:  {branch, target?, best_effort?}
  response: {success, manifest_xml?, error_message?}

Servers must ignore unknown options, so new options (e.g. criteria for
selecting a good build) can be added without another method.

The first new option is `best_effort`.  repo sets it when smart sync was
enabled implicitly by the manifest's sync-smartsync="true" default
rather than by -s.  The server may decline a best_effort request (e.g.
for rollout or cohort gating) by returning success=false.  The client
still owns the fallback policy: declined implicit requests fall back to
a regular sync, while explicit -s requests fail as before.

If the server does not implement GetApprovedManifestV2, repo falls back
to the legacy GetApprovedManifest call.

Bug: 545137090
Test: ./run_tests
Change-Id: I9297b728df26516012cb93dffc91d39c1fb0458f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/636623
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Rahul Yadav <yadavrah@google.com>
Tested-by: Rahul Yadav <yadavrah@google.com>

0 files changed, 0 insertions, 0 deletions