postgres: decouple the frontend-part

[gabor]

NOTE: this is the same PR as we did for mysql except i changed the word "mysql" to "postgres" .

fixes #87040

decouples the "frontend part"of the postgres-plugin.

how to test:
just run it, connect to a postgres datasoure, and make sure the query-editor still works.
(for grafana-people, just go to the ephemeral instance below, i configured it with a postgres datasource named psql you can use for testing)

[gabor]

/deploy-to-hg

[ephemeral-instances-bot]

• Preparing your instance. A comment containing your instance's url will be added to this PR when the instance is ready.
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[ephemeral-instances-bot]

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[mckn]

LGTM!