Add JsonFields for dynamic json field names

[emeroad]

Add JsonFields that can force the type of Key and Value.

Example

@Test
    public void testObjectKey() throws Exception {
        JsonFields.Builder<Key, String> builder = JsonFields.newBuilder(Key::name);  
        // default transform = Object::toString
        builder.addField(new Key("key1", 1), "value1");
        builder.addField(new Key("key2", 2), "value2");
        JsonFields<Key, String> jsonObject = builder.build();

        String s = mapper.writeValueAsString(jsonObject);
        System.out.println(s);
    }

    public record Key(String key, int value) {
        public String name() {
            return key + '-' + value;
        }
    }

{"key1-1":"value1","key2-2":"value2"}