Add occ command to repair mtime

[artonge]

occ command to repair files' mtime.

This is an early draft for another approach of fixing the files' mtime. It should be easier to update the mtime and to adapt it to different backends.

It's a modified version of the Scan command, so there is currently a lot of probably useless code.

This is to fix https://github.com/nextcloud-gmbh/server/issues/226#issuecomment-1054577115

[artonge]

@icewind1991 could you review the S3 part?

Notable changes:

• Added a headObject method in all implementation of IObjectStore
• Changed AmazonS3::headObject to a public method: https://github.com/nextcloud/server/pull/31386/files#diff-d26910d637d44440b2531cc06422315ff2c0553d6f14084c06b382dd6d10ac79R151
• Not sure what to return in StorageObjectStore::headObject: https://github.com/nextcloud/server/pull/31386/files#diff-ffe6a701709c4e9656d18bb0adf4cc63ec8b5c5fe02e970551fb6f7b713d01afR96