Introduce helper for dev/test TLS setup (backport #1999) - #2001

On branch mergify/bp/v5.x/pr-1999
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Changes to be committed:
	modified:   ci/start-broker.sh
	modified:   src/main/java/com/rabbitmq/client/ConnectionFactoryConfigurator.java
	modified:   src/main/java/com/rabbitmq/client/TrustEverythingTrustManager.java
	modified:   src/main/java/com/rabbitmq/client/impl/OAuth2ClientCredentialsGrantCredentialsProvider.java
	modified:   src/test/java/com/rabbitmq/client/test/PropertyFileInitialisationTest.java
	modified:   src/test/java/com/rabbitmq/client/test/ssl/TlsTestUtils.java
	modified:   src/test/java/com/rabbitmq/client/test/ssl/UnverifiedConnection.java

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   AGENTS.md
	both modified:   src/main/java/com/rabbitmq/client/ConnectionFactory.java
	both modified:   src/test/java/com/rabbitmq/client/test/ConnectionFactoryTest.java