Enhancement: Add upload checksum for Azure, GS, S3 and Swift backends

Previously only the B2 and partially the Swift backends verified the integrity
of uploaded (encrypted) files. The verification works by informing the backend
about the expected hash of the uploaded file. The backend then verifies the
upload and thereby rules out any data corruption during upload.

We have now added upload checksums for the Azure, GS, S3 and Swift backends,
which besides integrity checking for uploads also means that 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
