Add supported config options for additional datasources

[vmuzikar]

Description

Although Keycloak supports additional datasources (e.g. for user providers) which is also reflected in a quickstart, it does not provide a clearly supported way to do so. The only way right now is to use raw Quarkus options which we don't consider supported.

This issue should cover the following:

• Adding new config options to allow adding N additional config sources. This requires further discussion.
• Adding tests for additional config sources.
• Adding docs for it to the Server Development guide.

Discussion

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Motivation

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Details

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