Register keycloak classes for reflection only when keycloak-core present

[zakkak]

Co-authored-by: @essobedo

Fixes warnings appearing with #29886

[quarkus-bot]

Failing Jobs - Building 4ae0ad6

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

Full information is available in the Build summary check run.

Failures

⚙️ JVM Tests - JDK 17 MacOS M1 #

📦 extensions/resteasy-classic/resteasy-jackson/deployment

✖ io.quarkus.resteasy.jackson.LargeResponseTest. line 15 - More details - Source on GitHub

java.lang.NoClassDefFoundError: io/quarkus/resteasy/jackson/LargeResource
	at io.quarkus.resteasy.jackson.LargeResponseTest.lambda$static$0(LargeResponseTest.java:15)
	at io.quarkus.test.QuarkusUnitTest.lambda$withApplicationRoot$2(QuarkusUnitTest.java:204)

✖ io.quarkus.resteasy.jackson.MultipleTimeModuleTest. line 16 - More details - Source on GitHub

java.util.ServiceConfigurationError: io.quarkus.deployment.dev.CompilationProvider: Error accessing configuration file
	at java.base/java.util.ServiceLoader.fail(ServiceLoader.java:586)
	at java.base/java.util.ServiceLoader$LazyClassPathLookupIterator.parse(ServiceLoader.java:1180)

✖ io.quarkus.resteasy.jackson.NoMediaTypeTest. - More details - Source on GitHub

java.lang.RuntimeException: java.lang.IllegalArgumentException: /Volumes/actions-files/githubactions/actions-runner/_work/quarkus/quarkus/extensions/resteasy-classic/resteasy/deployment/target/quarkus-resteasy-deployment-999-SNAPSHOT.jar does not exist
	at io.quarkus.test.QuarkusUnitTest.beforeAll(QuarkusUnitTest.java:689)
	at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$invokeBeforeAllCallbacks$12(ClassBasedTestDescriptor.java:395)