Security manager test & fix

[SylvainJuge]

What does this PR do?

• fixes SecurityManager: security exception when agent tries to access proxy settings #3074
• adds systematic testing of security manager with a junit extension
• fix newly found similar issues

Sample stack trace when finding code calling the Security Manager without privileged action wrapping: here a call to System.getenv triggered indirectly by agent configuration sources.

Caused by: java.lang.SecurityException: missing privileged action in co.elastic.apm.agent.impl.ElasticApmTracerBuilder.getConfigSources(ElasticApmTracerBuilder.java:256)
	at co.elastic.apm.agent.util.TestSecurityManager.checkPrivileged(TestSecurityManager.java:190)
	at co.elastic.apm.agent.util.TestSecurityManager.checkPermission(TestSecurityManager.java:65)
	at java.base/java.lang.System.getenv(System.java:1152)          <----- this checks the security manager
	at org.stagemonitor.configuration.source.EnvironmentVariableConfigurationSource.reload(EnvironmentVariableConfigurationSource.java:50)
	at org.stagemonitor.configuration.source.EnvironmentVariableConfigurationSource.<init>(EnvironmentVariableConfigurationSource.java:19)
	at co.elastic.apm.agent.impl.ElasticApmTracerBuilder.getConfigSources(ElasticApmTracerBuilder.java:256)

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix

[ghost]

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Build stats

• Start Time: 2023-04-06T10:00:37.250+0000

• Duration: 13 min 50 sec

❕ Flaky test report

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[SylvainJuge]

making it ready for review to make it pass the integration tests

[github-actions]

/test