[COMMON] Enhance common/postgres to support operator based user and db creation

[COMMON] Enhance common/postgres to support operator based user and db creation

- add .postgresOperator.autoCreateUserSchema that can be set to true to automatically
  create a user-named schema inside the postgres db [0]
- add .postgresOperator.databaseInitSQL.key + .name to point the operator to an sql
  script inside a configmap [1]
- add .postgresOperator.users[].name to create the instance with certain users [2]
- add .postgresOperator.users[].databases[] to create the instance with certain dbs [3]
- add .postgresOperator.users[].options to grant a user certain role privileges [3]
- define a "common.postgresOperator.userSecret" function that returns the secret name
  that is created by the operator and that contains the user credentials.
  It works with the first entry of the .postgresOperator.user array for it's input params

[0] https://access.crunchydata.com/documentation/postgres-operator/latest/tutorials/basic-setup/user-management#why-is-the-schema-named-after-the-user
[1] https://access.crunchydata.com/documentation/postgres-operator/latest/tutorials/day-two/customize-cluster#initialization-sql-configmap
[2] https://access.crunchydata.com/documentation/postgres-operator/latest/tutorials/basic-setup/user-management#creating-a-new-user
[3] https://access.crunchydata.com/documentation/postgres-operator/latest/tutorials/basic-setup/user-management#creating-a-new-database

Issue-ID: OOM-3374
Change-Id: I3e64a71b3135a40f95531ea455b6084c330f0d6a
Signed-off-by: Fiete Ostkamp <fiete.ostkamp@telekom.de>