feat(backup): Enable export over RPC

[azaslavsky]

Exports are already done on a sequential, per-model basis, going in reverse dependency order. This change modifies that scheme to work across an RPC boundary: code running on REGION instances (the only kind of instance that may perform import/export operations) requests the exports using either the local implementation (if the model is REGION as well), or by sending an RPC call to the same method on the remote CONTROL silo instance. Code executed in MONOLITH mode (aka, self-hosted and single tenant instances) does all exporting locally.

The new import_export_service is thus a bit different from some of the others in hybrid_cloud. Whereas existing services like Access provide a means to get the same information using different logic depending on which Silo they are located in (ex: using Organization or OrganizationMapping as the source of truth), this service executes identical logic in all silo kinds, and requires users to pick the implementation themselves. That is, users must examine the model they are about to request be exported, and make their own decision as to which implementation, remote or local, to use. This is a bit unergonomic compared to other hybrid_cloud services, but it is only used by one rather unique endpoint (the import/export functions) for a very specific purpose, making this API a bit easier to swallow.

Issue: getsentry/team-ospo#196

[codecov]

Codecov Report

Merging #57740 (1bf2a99) into master (d4f5811) will increase coverage by 0.01%.
Report is 4 commits behind head on master.
The diff coverage is 85.71%.

❗ Current head 1bf2a99 differs from pull request most recent head ffc3ec2. Consider uploading reports for the commit ffc3ec2 to get more accurate results

@@            Coverage Diff             @@
##           master   #57740      +/-   ##
==========================================
+ Coverage   79.02%   79.03%   +0.01%     
==========================================
  Files        5130     5134       +4     
  Lines      223033   223269     +236     
  Branches    37566    37599      +33     
==========================================
+ Hits       176255   176466     +211     
- Misses      41140    41166      +26     
+ Partials     5638     5637       -1     

Files	Coverage Δ
src/sentry/backup/dependencies.py	91.35% <100.00%> (+1.31%)	⬆️
...ry/services/hybrid_cloud/import_export/__init__.py	100.00% <100.00%> (ø)
src/sentry/testutils/silo.py	84.91% <100.00%> (ø)
src/sentry/backup/helpers.py	95.34% <75.00%> (-4.66%)	⬇️
...try/services/hybrid_cloud/import_export/service.py	90.47% <90.47%> (ø)
...entry/services/hybrid_cloud/import_export/model.py	95.52% <95.52%> (ø)
...sentry/services/hybrid_cloud/import_export/impl.py	91.66% <91.66%> (ø)
src/sentry/backup/exports.py	81.57% <81.81%> (-3.37%)	⬇️
src/sentry/testutils/helpers/backups.py	89.36% <63.63%> (-8.01%)	⬇️

... and 6 files with indirect coverage changes

[hubertdeng123]

did a tentative scan and looks fine to me, but I'm lacking context. +1 to unblock