fix(rds): Infer cache secret namespace from pod environment

[MisterMX]

Description of your changes

This adds a function that infers the namespace of the RDS password cache secret from the following locations (in order):

1. POD_NAMESPACE env var
2. /var/run/secrets/kubernetes.io/serviceaccount/namespace file
3. fallback to crossplane-system

Until now crossplane-system was the hardcoded value.

Fixes #1834

Please note that this might cause a regeneration of passwords if the provider is running in another namespace than crossplane-system, but, due to the change, is now looking for the secret in another namespace where it does not exist.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

n.a.

[flaviomoringa]

@MisterMX I was waiting for this fix, but apparently its still not in the latest version, right?

[MisterMX]

I was waiting for this fix, but apparently its still not in the latest version, right?

We haven't backported this since it is a breaking change. It is going to be in the next minor release.