Review request for LOG4J2-2962

[Volkan Yazıcı]

In LOG4J2-2962, Magnus Kvalheim requests a functionality for the "map"
resolver similar to the one in "mdc". Guess what? They both operate on
`ReadOnlyStringMap`s and there goes my MR:
https://github.com/apache/logging-log4j2/pull/441 Here I merge the backends
of both resolvers into one: `ReadOnlyStringMapResolver`. Would anybody mind
taking a look, please?