Fixes #465

• uses log4j2 as implementation, which provides ability to set level at runtime
• redirects agent internal logger to slf4j, which then delegates to log4j
• this only provides a way to set the log level from within the agent, it does not yet allow to set the log level from user-controlled interface (for example remotely through opamp or a local configuration file).
• introduces elastic.otel.javaagent.log.level config option to set the log level using log4j levels it provides the ability to use OFF option to completely disable agent logging.