refactor(user-info-fetcher): move initialization into backends

[dervoeti]

Description

Optional follow-up for #779:

As @adwk67 mentioned in his comment, there were comments in the code about some necessary refactoring that might make sense once we add LDAP support. This is the PR that refactors the mentioned TODO items. In general, initialization logic was moved from main.rs into the specific backend, basically two things per backend:

• Reading the credentials
• Initializing the HTTP client for the backends that need it

We now have resolve functions that enrich the basic backend configuration from the CRD with the resolved data so we have it available as shared state in the get_user_info functions.

I ran the tests for Keycloak, OpenLDAP and AAS, they were all successful. There was no test for Entra as far as I know.
I couldn't find out how to run the ActiveDirectory integration test but I did not need to touch the AD backend anyway, because it doesn't need to resolve credentials. We could add a resolve function there as well that just copies the data, so the code looks more consistent with the other backends, but since I did not know how to run the integration test I preferred to leave it as it is. It's probably also fine this way.

The diff looks bigger than it is in the Github UI, the get_user_info functions for example were barely changed.

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