OSS Admin - rotate credentials

[Isan-Rivkin]

The problem

Currently in when using OSS (No ACL) there is no sane way on how to rotate the single admin credentials.
In cases of credentials leak or lost there one would need to rotate those credentials, without this option it's hard to rely on the server for long term.

Expected new behavior

Introduce (and document) a way rotating credentials.
To be clear, there's is single user - rotate those credentials (i.e admin with a single set of creds).
Can reuse $lakefs superuser and in the backend allow setting credentials.