Rewriting log appender

Allows to rewrite log levels of external appenders. A logback implementation of http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/rewrite/RewriteAppender.html

If testing is set to true, we will include the rewrite rules that are marked as testing only.

Usage:

<appender name="REWRITE_LOG_LEVEL" class="com.digitalasset.canton.logging.RewritingAppender">
  <testing>true</testing>
  <appender-ref ref="CANTON_TEST_LOG" />
  <rewrite class="com.digitalasset.canton.logging.Rewrite">
    <logger>io.grpc.netty.NettyServerStream</logger>
    <maxLevel>INFO</maxLevel>
  </rewrite>
  <rewrite class="com.digitalasset.canton.logging.Rewrite">
    <logger>com.digitalasset.canton.participant.ParticipantNodeInit:test/ParticipantRestartTest/participant/participant1</logger>
    <contains>Unclean shutdown due to cancellation in</contains>
    <exceptionMessage>test exception</exceptionMessage>
    <maxLevel>INFO</maxLevel>
    <testing>true</testing>
  </rewrite>
</appender>

Note that the logger name can be specific, including test names and other context info separated using colons or slashes, but can also just be the base logger name which then applies to all tests.