[services][backup] CreateBackup 3/3 - handle chunks and finish

Summary:
This adds remaining logic for CreateBackup handler: handling data chunks and finishing the request.

Depends on D6198

Test Plan:
The whole CreateBackupHandler can be tested for example this way:

cd services && yarn init-local-cloud
yarn run-blob-service-in-sandbox

# in another nix terminal
cd services/backup
RUST_LOG=backup=trace cargo run -- --port 50052 --sandbox --blob-service-url "http://localhost:50053"

Now the endpoint can be called either manually (e.g. BloomRPC) or by yarn run-integration-tests backup - these will fail yet, but the backup should be created, which can be checked by e.g. looking at backup and blob services logs or using aws-cli.

Reviewers: varun, tomek, jon, max

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6199