Add C-API Capsule

[Vizonex]

Warning

This pull request is a still a being worked on and is based off previous work I did for a different library based off #958

I have not added a whole lot yet in order to be sure that this is what other maintainers had in mind otherwise I will simply wait for input. It should be noted that this concept I had was based off unfinished work I had previously done for another library but works nevertheless.

Important

It should also be noted that getting this implemented will take time and planning on what the best approach would be to take and to do it all slow and steady. I have not implemented more objects besides Fields and Factories because I wanted to ensure that what gets added makes sense and is in the best interest of the maintainers in order to not make the process anymore stressful than it may already be to maintain.

Edit: Here is the objects I am planning to add to the C-API

Checklist

• Field
• Factory
• Ext types
• Raw Types
• StructMetaTypes (I'm planning to subclass SturctMeta to implement an SQLModel inspired class and Instrument attributes via subclassing Field Types (I'll make a PR for subclassing fields again since I closed the previous one))
• Struct Types
• Encoder/Decoder Types (Tools for extra customization would be nice)
• 1:1 Msgspec C Structures but renamed as Msgspec... Example: Field is named MsgspecFieldObject in msgspec.h so when someone compiles the code something Generic like Field (which could theoretically be used in a different C library or another Dev's implementation) it isn't being mixed and causing developers headaches and compilation issues.
• Cython Bindings

fixes #958

[Vizonex]

I'm going to try working on encoders today for a bit as well as planning out how users should subclass the encoder objects possibly using the C Structure and build around it as well as the writing in of bytes & bytearray types.

[jacopoabramo]

It would be nice if you could provide some documentation on how to use this feature. I'm surely curious myself

[Vizonex]

It would be nice if you could provide some documentation on how to use this feature. I'm surely curious myself

@jacopoabramo I got around to making the _testcapi module it should however be disabled on compiling for distribution It's based on my previous work from multidict but with a few improvements from it (mainly macros for adding the functions to the test module).

[Vizonex]

I'm going to fix test_field_new when I get back home from work today. But this is looking good so far...

[Vizonex]

@jacopoabramo I added Ext and StructMeta StructMeta Still needs testing but the main functions I wanted to add in are now complete should be obvious as to why I am including it but it all boils down to other SQL ORM libraries being slow and I have need to make my own using this library. I'll try coming up with a test example sometime later today when I get time off work to show how a user could go about using this new api in the _testcapi module.