feat(smalltrip,problems): http handler for problems (wip)

problem/problem.go

@@ -3,6 +3,7 @@ package problem
 import (
 	"encoding/xml"
 	"fmt"
+	"net/http"
 	"slices"
 )
 
@@ -43,6 +44,10 @@ func (p Problem) StatusCode() int {
 	return p.Status
 }
 
+func (p Problem) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	p.handler(p).ServeHTTP(w, r)
+}
+
 func WithType(t string) Option {
 	return func(p *Problem) {
 		p.Type = t