added FirestoreCollection internal parsing

[fenix-hub]

This PR will integrate automatic parsing/marshalling of FirestoreDocuments in FirestoreCollection functions.

Previously, to call FirestoreCollection management functions such as add(document_name, document) and update(document_name, document) it was necessary to create a FirestoreDocument and call a parsing function dict2fields(dictionary) in order to parse a GDscript Dictionary to a formatted body accepted by Firestore API (which is in the form of {fields: { var_type: {name:value} }}). In the same way, Firebase return bodies needed to be formatted to a Dictionary in order to make them more readable and usable in gdscript using the function fields2dict(document.fields).	Firebase	GDScript
{fields: {string: {"user":"user_name"}}}|{"user":"user_name"}

This PR will apply the following changes:

• dict2fields() and fields2dict() are now static functions, so if you want to call them, there's no need to create an empty FirestoreDocument anymore. Instead, they can be called like so: FirestoreDocument.dict2fields(dictionary)
• dict2fields will be automatically called inside add() and update() functions on FirestoreCollection objects. This way, to create or update a new document you will just need to pass a Dictionary like so: a_collection.add("", {"a_field":"a_value"})
• fields2dict will be automatically called upon receiving a FirestoreDocument from FirestoreCollection signals, and this value will be saved inside a FirestoreDocument doc_fields parameter. So, when receiving a document from a signal (for instance - calling the get(document_id) function on a FirestoreCollection), a dictionary of its fields will be put inside firestore_document.doc_fields as a Dictionary.
• calling print(document) on a FirestoreDocument will print an auto-formatted document, like this:

  doc_name: dWsg1Gt06cEpFE67o4U9, 
  doc_fields: {name:Another new name, number:Another new number}, 
  create_time: 2021-01-23T20:58:44.598972Z

[timjrobinson]

@fenix-hub looks like you accidentally committed project.godot changes in this PR. Might want to rotate those keys.

[fenix-hub]

@timjrobinson Thank you for noticing, I'll delete them even though it is not much of a problem for Firebase keys to be exposed.