Contribute Keycloak extensions to testcontainers-keycloak project

[snazy]

Contribute our customizations/additions for the Keycloak testcontainer to the testcontainers-keycloak project.

[snazy]

@adutra is this doable and not so much effort (refactoring et al)?

[adutra]

What could be easily contributed:

1. the utility methods to create realms, users and clients.
2. the utility methods to compute URIs.

I'm less sure about the configuration part (basically KeycloakConfig + constructor); it's slightly biased towards usage as a Quarkus test resource.

But I can give this a try.

[snazy]

It's not urgent - was just curious.