This diff leverages both ResponseBuffer and BlobStoredItem to create a generic stream that downloads the item from blob service, buffers the data and sends in chunks not exceeding the gRPC message size. A few notes:

• First message includes "additional extra info" - fields like attachment holders etc. Rest receive only ID and data chunk
• The data is buffered before sending, this is the solution 2 from https://phab.comm.dev/D4439#126984
• More data is not downloaded when the buffer is already filled enough
• Stream ends when there is no more data

The order of instructions is important here:

1. Push new data if not full
2. Finish if empty
3. Pop if not empty

Depends on D6243

At this point, the whole service can be tested with integration tests

cd services
yarn reset-local-cloud
nix run .#comm-blob
RUST_LOG=backup=trace cargo run -- --port 50052 --sandbox --blob-service-url "http://localhost:50051"
yarn run-integration-tests backup