Add a new repeated metadata field for revisions that contain multiple transactions

[josephschorr]

Reference: authzed/spicedb#2439

[ecordell]

why not do:

 repeated google.protobuf.Struct optional_transaction_metadata = 3;

?

I believe this change is wire-compatible

[barakmich]

It ought to be wire compatible, but it's worth testing first

https://protobuf.dev/programming-guides/proto3/#conditionally-safe-changes

[josephschorr]

It ought to be wire compatible, but it's worth testing first

https://protobuf.dev/programming-guides/proto3/#conditionally-safe-changes

Its wire compatible but will break all the generated clients