feat: support migrating an instance to sharding

[icewind1991]

• requires Fix metadata storage with sharding #48563
• Fix "File entry could not be inserted but could also not be selected with getId() in order to perform an update. Please try again." issue
• more testing

Support migrating existing instances to sharding.

Enabled by setting the sharding configuration and setting the from_shard_key and from_primary to the sharding configuration with the next storage/file id (so the first storage/file that will be created after sharding is configured).

The schema from the main db will need to be copied to the shard db's before enabling the sharding (having a command to help with this would be a nice stretch-goal)

Any files on the existing storages will still go to the main db, only newly created storages will be sharded.

[tcitworld]

Things seem to work, but in some cases, the "File entry could not be inserted but could also not be selected with getId() in order to perform an update. Please try again." error gets thrown. (creating a new user is the most reliable way I found).
While things seem to work otherwise, I suspect that this issue is a sign of a bigger connection handling issue.

I had tried #36313 a while ago

[come-nc]

Code looks sane, did not test.