Add logging support about spring-fu

[sdeleuze]

@tgirard12 Thanks for the latest improvements.

What do you think about providing a shortcut for LoggerFactory.getLogger(Foo::class.java) to have a more idiomatic syntax. We should not abuse of top level function but this one is going to be used widely. Maybe logger<Foo>() ?

[sdeleuze]

Ok to integrate logging to the core, but I would let logback support as an optional module and maybe use slf4j + slf4j-simple binding which would be sufficient for basic needs and consistent with Spring Fu goal to provide a minimal core. Also an optional log4j2 module would make sense.

[tgirard12]

Thanks for the proposal @liqweed, it seems to be a good idea, I will investigate on this too.

[tgirard12]

Also, I think we should integrate logging module into the core and provide by default a logback console appender with Info level.
It will provide a better first launch without all the Debug messages.

[liqweed]

About the logger<Foo>() syntax proposal - have you considered integrating with https://github.com/MicroUtils/kotlin-logging ?

It allows to define a logger (using examples from their README):
private val logger = KotlinLogging.logger {}

and call the logger (slf4j under the hood) using lambdas:
logger.debug { "Some $expensive message!" }

[sdeleuze]

@tgirard12 I don't think LogLevel in the logging module should import logback stuff, it should not require Logback JAR to work (it brokes the CI, see https://ci.spring.io/teams/spring-fu/pipelines/spring-fu).