Feature: Expose Context.methodsAllowed for use by MethodNotAllowedHandler

[flimzy]

The default MethodNotAllowed handler uses Context.methodsAllowed to set the "Allow" header in the response.

It would be nice to expose that list of allowed methods to a custom handler, as well.

Options I've considered:

1. Simply export that field of the Context struct. This would allow access when the Context value is retrieved from the r.Context()

2. Add a function that extracts the unexported field from a context, and returns it. Something like:

   func AllowedMethods(ctx context.Context) []string {
       rctx, ok := ctx.Value(RouteCtxKey).(*Context)
       if ok {
           return rctx.methodsAllowed
       }
       return nil
   }

3. Add a methodn on Router that takes a function that receives the list, and returns a handler. This seems really intrusive, though, and largely duplicates existing functionality:

   // CustomMethodNotAllowedHandler sets a custom MethodNotAllowed factory. Takes precidence over MethodNotAllowed if both are set
   func (mx *Mux) CustomMethodNotAllowedHandler(func(allowedMethods []string) http.HandlerFunc)

4. Re-calculate the list somehow. I don't see an efficient way to do this, but maybe I'm overlooking something.

5. As a last restort, I could potentially use a middeware that wraps the entire chi Mux, and detects the Allow: header to extract the values that way. Meh