Add a logback appender that can be added to a config to export context IDs

[anuraaga]

Heavily inspired by Armeria, even copied a file as-is

https://github.com/line/armeria/blob/master/logback/src/main/java/com/linecorp/armeria/common/logback/RequestContextExportingAppender.java

While MDC integration can cover non-logback / log4j, I think directly integrating with them like this and the log4j module work well. This is why I'm hoping to be able to inject logback.xml using auto instrumentation at some point though :)

/cc @trustin

[anuraaga]

I would generally copy the license header into the file I copied as well, but spotless deletes the second header if I try. For now I've added this NOTICE but let me know if there's something I should do here.

[iNikem]

Should you update some documentation as well? At least README.md or some sort of highly visible link to module specific readme

[anuraaga]

I was thinking of adding to top-level README.md but since this isn't supported by agent yet thought it might be confusing. How about I struggle try to set up auto instrumentation in a followup and otherwise will start a dedicated section for manual-only instrumentations we provide.

[anuraaga]

@zenmoto FYI I've added this logback instrumentation very similar to the log4j instrumentation. Happy to update if there's any semantic changes :)