Create more than one user and database when configured

[marcel-wollschlaeger]

This PR changes the stackit-postgres-db module in the following ways:

• allows the creation of multiple databases with different names than the instance
• allows the creation of a root user with a different name than the instance
• allows the creation of multiple additional users with Login Permission
• creates a local_file resource for the manifest if secrets are managed
• Test and Readme adjustments

[HartmannVolker]

After a detailed review I noticed that we need to add some more validation. If we use the local_file provider, terraform will keep the control over the generated file. This mean any manual change to the file will result in a diff in terraform.

[marcel-wollschlaeger]

@HartmannVolker I've added a config-map output and applied all the fixes we talked about.