Support for slog

[vorner]

We have a crate (spirit-log) for configuring the „classical“ log framework. There's also slog and it would be great if we could support that one too.

As slog doesn't really want to have a global logger, the pipeline should take a handle into where it would install the loggers (or, drains in slog terminology). The handles probably can come from slog-atomic.

The first part of the task would be to search for the right crates to build this on (spirit-log uses fern as the backend, there probably are some crates that allow building rich configurations for slog too). Then the rest is to get inspired a bit by spirit-log and create similar-looking (but possibly slightly different) crate.

As some additional features (which can be added in a follow-up branches) I believe we might want to have some per-logger filtering based on value of specific tags (eg. if log messages get tagged by IP address of a client, we might want to pull out a separate log file for a specific IP address).

I'll offer help to whoever takes this, in form of guidance, reviews, lightweight mentoring (limited by the amount the amount of free time I find).

If you want to work on this, please write a comment that you claim it.