Why was this needed

Support for an external PostgreSQL instance is implicit, but confusing to configure. Users still have to deploy the postgresql-ha subchart and just use a serviceName pointing to an external PostgreSQL.
Specifying credentials is done via values.yaml or --set at the command line, which could be insecure, depending on who else has access to the server. Using existing Secrets is a pretty common pattern for this.

What was implemented

• allow disabling postgresql-ha
• add keys and templates for rendering credentials and connection details for external PostgreSQL
• reuse existing secret approach for Cassandra as used in subchart

How it was tested

Ran helm template with the following overriding values:

cassandra:
  enabled: true
  dbUser:
  #   user: setplease
  #   password: setplease
    existingSecret: test-cassandra-secret

postgresql-ha:
  enabled: false

externalPostgres:
  host: localhost
  port: 5432
  database: thingsboard
  # existingSecret:
  #   name: test-postgres-secret
  #   username: username
  #   password: password