Add first version of Keycloak admin client based on Reactive REST Client

[geoand]

Alternative to #24294 that requires a new Keycloak release

[cescoffier]

Wasn't the last release of keycloak enough?

[geoand]

It introduced the changes we needed. This is the new extension that uses the changes and leverages the Reactive REST Client under the hood

[cescoffier]

Ah ok, I misundertood.

[cescoffier]

We would need to document that somewhere.
We also do not have any tests for the non-reactive version (I think @sberyozkin was working on that), but we would need some tests for this version.

The rest looks great!

[geoand]

I added a couple mentions in the security docs.
Furthermore, it has already been mentioned in https://github.com/quarkusio/quarkus/blob/main/docs/src/main/asciidoc/resteasy-reactive-migration.adoc#keycloak-admin-client

[sberyozkin]

LGTM, thanks @geoand

[geoand]

YW!

[sberyozkin]

Hi @geoand @cescoffier integration-tests/keycloak-authorization was the only one which was using the non-reactive admin client - a few of OIDC integration tests were doing it it was slowing down the native builds as the admin client was bringing a lot of extra dependencies so in the end it was only integration-tests/keycloak-authorization which was using it. This PR has changed it and now the reactive admin client is used - it is fine;
I'll update one of other OIDC tests to use the non-reactive admin client for completeness with one of the next Keycloak related updates

I think the doc comment was addressed. As for the tests, Sergey will deal with them later AFAICS.

[Foobartender]

Quick feedback: I tried it and got a 404 error. It tries to access the URL below, where the last path element ("roles") is duplicated:
http://localhost:9080/auth/admin/realms/myrealm/roles/roles?search=myrole&briefRepresentation=true

I'll look at it closer and create an issue as soon as I find the time.

Edit: has been fixed (#24991, #25028).

[rvansa]

I wonder, can we expect a client that uses Mutiny in the API?

[geoand]

That's one for @sberyozkin and @pedroigor