[ENHANCEMENT] create a database key for external psql db for webmodeler restapi

[hamza-m-masood]

Describe the use case:

Currently, it is possible to define the database in the external psql instance for identity and keycloak:
identity.externalDatabase.database
identityKeycloak.externalDatabase.database

But it is not possible to define the database for web-modeler rest API. It would be nice to have an entry like this so customers can comfortably use one existing psql instance with multiple DBs inside.

the web-modeler rest API expects you to have created the web-modeler database first before installing the Helm chart, and there is no way to give a different name.

Desired outcome and acceptance tests:

Having an entry in the values.yaml like this would be sufficient:
webModeler.restapi.externalDatabase.database

PR: #3614

[bkenez]

Draft PR ready:
#3614
Added the value, and included it in the schema and readme, was not sure if I should include it in the helper function that builds the jdbc URL, did it anyway, but can revert that part if it's not needed.