Enhancement: Add upload checksum for azure, gs, s3 and swift backends

Up to now only the B2 backend and partially the Swift backend verified the
integrity of uploaded (encrypted) files. The verification works by informing
the backend about the expected hash of the uploaded file. This allows the
backend to verify the upload and thereby rules out any data corruption during
upload.

We have added upload checksums for the azure, gs, s3 and swift backends.

Restic can now be used to store backups in S3 buckets which have Object Lock
enabled.

https://github.com/restic/restic/issues/2202
https://github.com/restic/restic/issues/2700
https://github.com/restic/restic/issues/3023
https://github.com/restic/restic/pull/3246