Upload protobuf encoding of snapshot/updates to bulk storage.

[mrdziuban]

Fixes #6497

This updates SingleAcsSnapshotBulkStorage.getSource and UpdateHistorySegmentBulkStorage.getSource to upload both the API and protobuf encoding of the relevant data to bulk storage.

After incrementing the contracts/updates count metrics, each class:

• Fans out for both encodings
• For each encoding, encodes and uploads the data, then increments object count metrics
• Fans back in so the resulting Source contains the keys of all uploaded objects for both encodings

This Pekko logic is handled by a new generic MultiEncodingBulkStorageFlow, which uses Pekko's Broadcast and Merge to handle the fan-out and fan-in behavior.

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[mrdziuban]

Note I haven't yet tackled this point from #6497

• Add internal APIs to read the protobuf encoded objects (could wait until we actually need those when we implement DB backfilling)

Let me know if you'd like to see those new APIs be a part of this PR, or would prefer to hold off on them. If I should include them, would it be okay to update the existing /v0/history/bulk/acs and /v0/history/bulk/updates endpoints to add an encoding parameter, or would you prefer to see new endpoints specific to the protobuf encoded objects?