[SUREFIRE-1639] Add ability to configure JUnit 5 TestExecutionListener

[pan3793]

Issue: #2122

Description

The listener provider property now accepts JUnit Platform
TestExecutionListener classes in addition to JUnit 4
org.junit.runner.notification.RunListener classes.

For each comma-separated entry, whitespace is trimmed and empty entries are
ignored. The provider then tries, in order:

1. JUnit Platform TestExecutionListener (instantiated directly and registered
   with the Launcher)
2. JUnit 4 RunListener (wrapped in JUnit4ListenersAdapter)
3. otherwise ignored by this provider (TestNG listeners are forwarded to the
   TestNG engine via testng.listeners)

Changes

• JUnitPlatformProvider resolves each listener entry as a JUnit Platform
  TestExecutionListener or a JUnit 4 RunListener, trimming whitespace and
  skipping empty entries.
• Documents the new Registering a custom TestExecutionListener section in
  maven-surefire-plugin/src/site/markdown/examples/junit-platform.md.vm.
• Adds unit test testExecutionListenerIsConfiguredViaListenerProperty and the
  junit5-testexecutionlistener integration test with
  JUnit5TestExecutionListenerIT.

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-surefire-plugin</artifactId>
  <configuration>
    <properties>
      <property>
        <name>listener</name>
        <value>com.example.MyTestExecutionListener</value>
      </property>
    </properties>
  </configuration>
</plugin>

How it was tested

• mvn -pl surefire-providers/surefire-junit-platform test -Dtest=JUnitPlatformProviderTest - 48 tests pass
• mvn -pl surefire-providers/surefire-junit-platform checkstyle:check -
  no violations
• Integration test JUnit5TestExecutionListenerIT verifies end-to-end that
  the configured listener runs in a forked test JVM

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[pan3793]

the test failure looks to be caused by a transient network issue

[pan3793]

@olamy could you please take a look at this one too? thank you in advance