Add full support for the SLF4J fluent logging API

[karstenspang]

This implements #407. Changes:

• The TestLogger class implements the LoggingEventAware interface. This means that it has direct access to the logging event produced by the fluent logging API, so it can store it more correctly. This breaks compatibility with SLF4J versions prior to 2.0.0.
• The LoggingEvent class supports multiple markers. This is allowed by the fluent API.
• The LoggingEvent class stores the key/value pair list that can be generated fluent API.
• The toString, equals, and hashCode methods of the LoggingEvent class have been updated to match the two changes above.
• The new TestLoggingEventBuilder class can be used for fluently creating LoggingEvent instances for use as expected values. That method is necessary if the expected event has key/value pairs or multiple markers. The alternative would have been to exponentially add even more convenience build routines to LoggingEvent. The existing ones are kept unchanged.
• Test cases that rely on the exact format of the output of LoggingEvent.toString() have been adapted.

BTW, I realized that there is nothing that prevents a logging event from having a null message, but LoggingEvent.toString() did not handle that. I have fixed it.

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[valfirst]

@karstenspang thank you for your contribution!