Consider moving logging handlers to non ref structs

[333fred]

Thanks for this library, I'm quite liking using it. However, I'd like to request that the interpolation handlers be changed to regular structs, not ref structs. As far as I can tell, there's no nested ref struct usage in the handlers, so there's no benefit for them being marked as ref structs. This blocks their usage with interpolation arguments that are await expressions (which, yes, it can certainly be argued that logging shouldn't have nested asyncs, but given that interpolation won't execute it if the log level isn't enabled I think this case is fine).