Add text format representation

[Arman17Babaei]

Google protobuf provides a library to parse and create text representations of messages, google.protobuf.text_format to be specific. I think it would be a good idea to add such module to this project for compatibility.

The text representation this module creates, basically looks a lot like json. Some differences are:

• Field names are not quoted
• Repeated objects are actually repeated in the structure, and are not an array

[zhuoqun-chen]

Hi @Arman17Babaei , did you solved this problem yet? I'm having the same use case and wanna load *.txtpb using betterproto objects.