Add support for values in MDC in JSONLayout KeyValuePairs

The new KeyValuePair within JSONLayout as implemented for https://issues.apache.org/jira/browse/LOG4J2-1694 doesn't provide support for MDC stored values.

In PatternLayout one can do something like this:

<Pattern>%d{ISO8601}{UTC} %d{Z}{UTC} [CID:%X{conversation-id}] [%t] %-5p %c{1.} - %m%n</Pattern>

But it seems that

<KeyValuePair key="CID" value="%X{conversation-id}"/>

or even

<KeyValuePair key="CID" value="${conversation-id}"/>

doesn't work.

Variable replacement doesn't appear to read from the MDC.