Enhance KeycloakTestClient to support a client_credentials grant

[sberyozkin]

Fixes #29933.

This PR adds 4 getClientAccessToken methods making it easy to get the access token using a client_credentials grant from a default realm set up by DevServices for Keycloak, and 4 getRealmClientAccessToken to do the same but also letting users set a realm name as one of the method parameters.
The same pattern is used with the existing getAccessToken/getRealmAccessToken methods where a password grant is used.
Also tried to improve the description of how various properties are set.
Update OidcTokenPropagationTest - in the existing test the propagated token contains an alice name because the token was acquired using a password grant. A new test method acquires a token using a client_credentials grant for a quarkus-app client, Keycloak represents it as a service-account-quarkus-app in the token issued using client_credentials grant

CC @pedroigor

[quarkus-bot]

Failing Jobs - Building e8f33b3

Status	Name	Step	Failures	Logs	Raw logs
✔️	JVM Tests - JDK 11
✔️	JVM Tests - JDK 17
✖	JVM Tests - JDK 17 MacOS M1	Set up runner	⚠️ Check →	Logs	Raw logs
✔️	JVM Tests - JDK 18

[gsmet]

Not backporting because based on work that is not marked for backport.