Enhancement: Support B2 API keys restricted to hiding but not deleting files

When the B2 backend does not have the necessary permissions to permanently
delete files, it now automatically falls back to hiding files. This allows
using restic with an application key which is not allowed to delete files.
This can prevent an attacker from deleting backups with such an API key.

To use this feature create an application key without the `deleteFiles`
capability. It is recommended to restrict the key to just one bucket.
For example using the `b2` command line tool:

`b2 create-key --bucket <bucketName> <keyName> listBuckets,readFiles,writeFiles,listFiles`

Alternatively, you can use the S3 backend to access B2, as described
in the documentation. In this mode, files are also only hidden instead
of being deleted permanently.

https://github.com/restic/restic/issues/2134
https://github.com/restic/restic/pull/2398
