Support for user id migration

[CarlSchwan]

For many reasons it can be helpful to support renaming an user_id

Implementation:

• Add a event that app can listen to, and use to rename the user id in their table
• Add a feature flag in appinfo file, so that there can be a warning when we are doing a migration and one app is enabled which doesn't support it

[susnux]

For many reasons it can be helpful to support renaming an user_id

What would be such a reason? I think it brings more problems than it solves.
We should instead move to a fully internal userid, which is never exposed to users.
Ref: #57597

So everything plugins into that UUID / snowflake for a user.
E.g. SAML / LDAP / OIDC then can map the selected login attribute as a login name to the userid in that table.
Meaning user or app provided names are never used for userids but mapped first to the Nextcloud generated userid.

[SystemKeeper]

• Also ref Change/rename login name #5488