Add PyJWT._{de,en}code_payload hooks

[akx]

This enables custom subclasses of PyJWT to customize decoding of the payload data of the JWS.

There's an example on how to subclass the function in the test case.

Refs #666 (reimplementation of it)
Refs #753 (reimplementation of it, borrows test case)

[russoale]

Alternatively, this could be implemented as well by passing a callable

    def decode(
        self,
        jwt: str,
        ...
        # hook for custom payload decompression 
        decompress_payload: Optional[Callable[Dict]] = None,
        ...
        **kwargs,
    ) -> Dict[str, Any]:

   # passing to 

    def decode_complete(
        self,
        jwt: str,
                ...
        # hook for custom payload decompression 
        decompress_payload: Optional[Callable[Dict]] = None,
        ...
        # kwargs
        **kwargs,
    ) -> Dict[str, Any]:

Then checking if the function has been passed and use it or proceed with default

[akx]

@russoale Possibly, but those are pretty arg-heavy functions as-is. Also, "decompress" is not a good name – the function can do something else than just decompress.

[simon-sk]

@akx Would it be possible to add a similar hook for the encode function as well?

[akx]

@simon-sk Good idea. Added.