Strengthen Debezium platform helm chart [DBZ-9324]

Migrated from [DBZ-9324](https://issues.redhat.com/browse/DBZ-9324)

hello,
i have some suggestions which stem from my try to use this helm chart (with external db)
couple of problems with helm values:
{code:java}
domain:
  url: dbz.my.sv
database:
  enabled: false
  name: debezium
  host: postgresql.infra
  # port: 5432  <----- there is no port support here what if my db is on another port?
  auth:
    existingSecret: debezium-postgresql  #<------- if username and password is set this SHOULD be left empty but it's not possible {code}

and one more important thing i had to go through the code and read the database charts to see that i need to encode my username/password in secret to base64 before using, it's a good idea to append this to the description of the values in the docs.

Start by reading the Helm chart values and database chart referenced in the issue, then inspect how the external database host, port, and authentication settings are handled. Add support for a custom database port, clarify the interaction between existingSecret and explicit credentials, and document the required secret encoding in the values documentation. Verify the chart with an external PostgreSQL database configuration.

Written by the indexing model from the issue text.