manifest: Add sync-strategy attribute to project elements

The only supported sync-strategy is "stateless". The intent is to keep
the local workspace as small as possible by not keeping history during
syncs. This prevents disk space waste for projects with large binaries
where we only care about the current version.

A follow up change will implement the logic.

Bug: 498730431
Change-Id: I84a436a9ca2492893163c6cfda6c28dc62a568f0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/568462
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

project.py

@@ -558,6 +558,7 @@ class Project:
         sync_s=False,
         sync_tags=True,
         clone_depth=None,
+        sync_strategy=None,
         upstream=None,
         parent=None,
         use_git_worktrees=False,
@@ -610,6 +611,7 @@ class Project:
         self.sync_s = sync_s
         self.sync_tags = sync_tags
         self.clone_depth = clone_depth
+        self.sync_strategy = sync_strategy
         self.upstream = upstream
         self.parent = parent
         # NB: Do not use this setting in __init__ to change behavior so that the