Better logging for replaygain plugin

[RollingStar]

After some discussion here, I think we should do this:

https://github.com/beetbox/beets/blob/e10b955a931e4c205b0cadf0860797c0aeee736c/beetsplug/replaygain.py#L1163C14-L1163C14

1. Add a check for true peaks
2. warn if true peaks is not supported by the backend
3. log one time on init whether it's true peaks or sample peaks

warning - Your config specifies true peaks which only work in the ffmpeg backend.
warning - The command may work as-is, but we recommend changing your backend to "ffmpeg" or your config to "sample". info Analyzing true peaks (slower, more accurate).
info Analyzing sample peaks (faster, less accurate).

The plugin unfortunately specifies true peaks on init if the config is not defined. I can't think of a clean way to default to sample for the others but truepeaks for ffmpeg. (Specify it once in the overall init then again when ffmpeg init happens? Does that even work?)

I can do the PR once we agree on approach.