(v3 ndc-postgres) Database connection string based on JWT

### Component

c/v3-ndc-postgres

### Is your proposal related to a problem?
No

### Describe the solution you'd like
Currently Hasura v2 provides possibilities of changing the connecting database based on JWT variables (connection templating). In order for us (enterprise Hasura user) to hop over to v3 we require these functionalities.

Simply said based on a specific value in the JWT it should be capable of switching to another connection string. All databases have the same exact schema, only data differs

The issue identifies c/v3-ndc-postgres as the component and compares the requested behavior with Hasura v2 connection templating. Begin by locating the v3 Postgres connection-handling entry point and the JWT variable flow; done means selecting a connection string from a JWT value while preserving the shared-schema behavior.

Written by the indexing model from the issue text.