Feature: PostgreSQL should support Managed Identity and have the ability to create AAD users

[matthchr]

Items:

1. PostgreSQLServer should have a PostgreSQLServerAdministrator subresource, allowing customers to delegate user creation permissions to the managed identity ASO is running as. (see MySQLServerAdminstrator for an example of this pattern).
2. PostgreSQLUser should have an AAD variant PostgreSQLAADUser (see MySQLAADUser for an example of this pattern).

This would allow customers to configure the operator to create users in a namespace other than the namespace the PostgreSQLServer was in (as requested in #1396). This also would enable the same AAD/Managed identity scenarios that MySQL already supports.

[matthchr]

Planning on doing this in ASO v2, but it hasn't happened yet.

[matthchr]

We still want to do this

[matthchr]

We still want to do this, but it's also blocked behind #2834

[theunrepentantgeek]

#2834 has completed; this is still something we want to do.

[matthchr]

Still something we're interested in, but user interest seems low currently.