Add LogRecorder utility

[strangelookingnerd]

When working on jenkinsci/configuration-as-code-plugin#2623 I was looking for a way to make use of LoggerRule without depending on JUnit 4.
My workaround jenkinsci/configuration-as-code-plugin#2623 (comment) does the trick but I think it should be optimized.

This PR extracts the logic from LoggerRule and provides a JUnit-version-agnostic implementation that is more versatile.

I was thinking of adding a JUnit5 extension that is similar to LoggerRule however I do not really see the point.
The only thing that @Rule public LoggerRule rule = new LoggerRule(); does is to have an automated clean-up in the TestCase#after. The way to do this way sleeker in my opinion is to make use of AutoClosable like so:

@Test
void myTestCase() {
  // do stuff that is required for my test...
  try (LogRecorder recorder = new LogRecorder().record("Foo", Level.INFO).capture(100)) {
    // do the stuff I want to record...
    assertThat(recorder, LogRecorder.recorded(equalTo("MyMessage")));
  }
}

See LogRecorderTest for more examples of this.

Ultimately this new utility enables more tests to migrate away from JUnit4.

Testing done

Clean build with mvn clean verify

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