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

parser/lowercase_camel_json.go

@@ -107,7 +107,7 @@ func (c ReplacingJSONMarshaller) MarshalJSON() ([]byte, error) {
 		var removeZeroVAlues func(m map[string]any)
 		removeZeroVAlues = func(m map[string]any) {
 			for k, v := range m {
-				if !hreflect.IsTruthful(v) {
+				if !hreflect.IsMap(v) && !hreflect.IsTruthful(v) {
 					delete(m, k)
 				} else {
 					switch vv := v.(type) {