Allow external resources secret key customization

Is your feature request related to a problem? Please describe.
The ability to define the keys in the secret for external db and cache instances.

A lot of operators for services like PostgreSQL, example CloudNativePG https://cloudnative-pg.io/documentation/1.27/applications/#secrets, automatically generate secrets that contain all the needed information for Pulp to connect to the DB but with different keys than what the Pulp Operator expects.

Example CNPG secret

apiVersion: v1
kind: Secret
metadata:
  name: app-db-app
  namespace: app
type: kubernetes.io/basic-auth
data:
  dbname: app
  host: app-db-rw
  jdbc-uri: jdbc:postgresql://app-db-rw.app:5432/app?password=password&user=app
  password: password
  pgpass: |
    app-db-rw:5432:app:app:password
  port: "5432"
  uri: postgresql://app:password@app-db-rw.app:5432/app
  user: app
  username: app

Describe the solution you'd like
The ability to set the key names for the values.

Example:

spec
  database:
    external_db_secret: my-db
    keys:
      host: host
      name: dbname
      port: port
      username: username
      password: password

Describe alternatives you've considered

• Use External-Secrets to sync a secret from an external source to both formats. This requires using an external source which is another attack point that requires securing which if it's a service entirely based in the cluster is unnecessary.
• Use External-Secrets to watch the generated secret and create a new secret.
• Manually copying the secret. Not conducive to automation, doesn't scale

All the methods require maintaining multiple secrets and ensuring they remain in sync.

Additional context
Add any other context about the feature request here.

No files or tests are named in the issue. Start by locating the operator's external database and cache secret handling and the configuration schema for external resources. Done means users can map the expected connection fields to differently named Secret keys, with coverage for the PostgreSQL-style example.