Helpers for creating simple/stub versions of `SelfAwareStructuredLogger` and `LoggerFactory`

[bplommer]

This came up in discussion on https://github.com/http4s/http4s/pull/6629 about whether the http4s logging middleware should continue to allow the user to provide String => F[Unit] instead of LoggerFactory - allowing it adds implementation complexity, but currently it takes a tremendous amount of boilerplate to lift a String => F[Unit] into a LoggerFactory. cc @armanbilge

Names and access modifiers subject to bikeshedding of course.

[rossabaker]

Did a courtesy merge because we let this rot when it was already close. I didn't follow all the spidering conversations, but I think this was ready if we made the helper traits private to reduce the bincompat surface?

[morgen-peschke]

currently it takes a tremendous amount of boilerplate to lift a String => F[Unit] into a LoggerFactory

If this is still a pain point, #854 may help and would benefit from your input