Enhancement: Add config option to set S3 storage class

The `s3.storage-class` option can be passed to restic (using `-o`) to
specify the storage class to be used for S3 objects created by restic.

The storage class is passed as-is to S3, so it needs to be understood by
the API. On AWS, it can be one of `STANDARD`, `STANDARD_IA`,
`ONEZONE_IA`, `INTELLIGENT_TIERING` and `REDUCED_REDUNDANCY`. If
unspecified, the default storage class is used (`STANDARD` on AWS).

You can mix storage classes in the same bucket, and the setting isn't
stored in the restic repository, so be sure to specify it with each
command that writes to S3.

https://github.com/restic/restic/pull/2220
https://github.com/restic/restic/issues/706
