GODRIVER-1765 Improve JSON marshaling/unmarshaling for bson.D, bson.M and bson.A.

[qingyang-hu]

GODRIVER-1765

Summary

This PR adds json.Marshaler and json.Unmarshaler for primitive.D, so it can handle JSON objects properly.
The PR also adds json.Unmarshalers for primitive.M and primitive.A, so JSON strings can be unmarshaled into nested structures correspondingly.

After the "primitive" merging into "bson" package, we can migrate these primitive methods to their bson types.

[mongodb-drivers-pr-bot]

API Change Report

./bson/primitive

compatible changes

(*A).UnmarshalJSON: added
(*D).UnmarshalJSON: added
(*M).UnmarshalJSON: added
D.MarshalJSON: added

[prestonvasquez]

LGTM!

[prestonvasquez]

[Optional] You could combine the logic in D.UnmarshalJSON and m.UnmarshalJSON using generics:

func newJsonObjDecoder[T D | M](b []byte, val *T) (*json.Decoder, error) {
	dec := json.NewDecoder(bytes.NewReader(b))

	t, err := dec.Token()
	if err != nil {
		return nil, err
	}

	if t == nil {
		*val = nil
		return nil, nil
	}

	if v, ok := t.(json.Delim); !ok || v != '{' {
		return nil, &json.UnmarshalTypeError{
			Value:  tokenString(t),
			Type:   reflect.TypeOf(*val),
			Offset: dec.InputOffset(),
		}
	}

	return dec, nil
}