MINOR: Move JaasModule into JaasUtils

[dino2895]

This PR moves JaasModule into JaasUtils as a nested record,
alongside JaasSection.

It also updates KafkaClusterTestKit to reference the nested
JaasUtils.JaasModule type and removes the standalone JaasModule.java
file.

This is a code organization change with no intended behavior change.

Testing

./gradlew clean :test-common:test-common-runtime:test \
  --tests
'org.apache.kafka.common.test.junit.ClusterTestExtensionsTest.testSaslPlaintext'
\
  --tests
'org.apache.kafka.common.test.junit.ClusterTestExtensionsTest.testSaslPlaintextWithController'
\
  --tests
'org.apache.kafka.common.test.junit.ClusterTestExtensionsTest.testSaslSslWithController'

Reviewers: Chia-Ping Tsai chia7712@gmail.com

[chia7712]

Do you like JaasUtils.JaasModule.plainLoginModule? If not, could you move plainLoginModule out of JaasModule scope?

[dino2895]

JaasUtils.plainLoginModule is clearer. I’ll move the factory method out of the nested JaasModule scope.

[dino2895]

I moved plainLoginModule out of the nested JaasModule scope, so the call site now uses JaasUtils.plainLoginModule(...).