Add a function to inject a logger into a context

[ernesto-jimenez]

Right now, clue/log is the only package that can define a middleware like log.HTTP, since ctxLogger is unexported.

It's feasible other people might have to develop their own middlewares, so it would be good to have something like:

log.WithContext(requestCtx, loggerCtx)

// Maybe include this too:
log.MustContainLogger(ctx)

// within clue
func MustContainLogger(logCtx context.Context) {
  	l := logCtx.Value(ctxLogger)
	if l == nil {
		panic("provided context without log.Context()")
	}
}

It would allow things like:

func WorkerLog(logCtx context.Context) func(worker.Handler) worker.Handler {
	...
}

[raphael]

Makes sense! Left a few comments in the commit, we can merge once these are addressed.