feat: Add UpdateSyncTarget to registry #1346
                
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Why this should be merged
This adds a lot of the synchronization logic necessary for ensuring UpdateSyncTarget can only be called at the correct times. Note that deciding when to call UpdateSyncTarget is not within scope for the registry.
Closes #1343
How this works
Tracks the current target for the syncers, as well as adds stubs for the UpdateSyncTarget implementations (see comments) and the finalization logic #1344
How this was tested
New UT
Need to be documented?
No.
Need to update RELEASES.md?
No.