Add `LoggersEndpoint` that can enables listing and configuration of log
levels. This actuator builds on top of the `LoggingSystem` abstraction
and implements support for Logback, Log4J2, and JUL.  The LoggingSystem
interface is modified to require each implementation to list the
configuration of all loggers as well as an individual logger by name.

The MVC endpoint exposes these behaviors at `GET /loggers` and
`GET /loggers/{name}` (much like the metrics actuator).

In addition `POST /loggers/{name}` allows users to modify the level for a given
logger. This modification is passed to the logging implementation, which
then decides, as an internal implementation detail, what the final outcome
of the modification is (e.g. changing all unconfigured children). Users
are then expected to request the listing of all loggers to see what has
changed internally to the logging system.

Closes gh-7086