Add ContentTypes to config

This is an empty struct for now, but we will most likely expand on that.

```
[contentTypes]
  [contentTypes.'text/markdown']
```

The above means that only Markdown will be considered a content type. E.g. HTML will be treated as plain text.

Fixes #12274

common/hreflect/helpers.go

@@ -74,6 +74,16 @@ func IsTruthful(in any) bool {
 	}
 }
 
+// IsMap reports whether v is a map.
+func IsMap(v any) bool {
+	return reflect.ValueOf(v).Kind() == reflect.Map
+}
+
+// IsSlice reports whether v is a slice.
+func IsSlice(v any) bool {
+	return reflect.ValueOf(v).Kind() == reflect.Slice
+}
+
 var zeroType = reflect.TypeOf((*types.Zeroer)(nil)).Elem()
 
 // IsTruthfulValue returns whether the given value has a meaningful truth value.