Provide a way to provide your own MDCAdapter

[patrickjamesbarry]

We are trying to use LogbackServiceProvider, but provide our own MDCAdapter that extends LogbackMDCAdapter. However, because of the way LogbackServiceProvider news up a its own LogbackMdcAdapter (in the initialize method), and does not use a 'getter' when referencing it, it prevents devs from being able to override the MdcAdapter. We see a dependency on the MDCAdapter has to be an instanceOf LogbackMdcAdapter, which should be ok for us, since ware extending that class.

This forces a dev to have to create an entire implementation of SLF4JServiceProvider, when all they want to do is provice a custom MDCAdapter

Also reported: https://jira.qos.ch/browse/LOGBACK-1496