Feature Request - Allow `PostgresCluster` to extract `spec.authentication.rules[].options` values from `Secret` manifest

[Amphaal]

Right now, when trying to allow my LDAP users to access my postgres databases, I need to provide both LDAP bind user and password like this:

... which is clearly not secure. A cleaner way of doing this would be to mimic what backups.pgbackrest.configuration[].secret allows us;

Here's what I had in mind =>

It's kind of an edge case, and I think only the RADIUS method users would benefit from this also.

https://docs.postgresql.fr/12/auth-ldap.html
https://docs.postgresql.fr/12/auth-radius.html

Does it make sense ? Are there alternatives I have not considered (like using patroni) ?