* Move the legacy JUL methods from LogBase to BaseJulLogger.
* Add a JavaLoggingAdapter as the general JUL adapter for the Logger SPI, which makes it convenient to use (as the default logger).
* Add LoggerSpiProvider to resolve Logger SPI.
* Polish the logback demo.

Signed-off-by: Eric Zhao <sczyh16@gmail.com>

* There are two types of logger for command center and common modules specified by  annotation of `LogTarget`
* Add implementing examples in `sentinel-demo/sentinel-demo-log-logback`
* All implementations should support placeholder '{}'