PYON: add plugin support

[drewrisinger]

Overview

This PR adds plugin support for PYON, allowing PYON to be extended with more complex data types
at the user's discretion.

Use Cases:

• Adding complex data types that would otherwise need to be reduced to tuples/dicts, for storage in e.g. ARTIQ results or sent across networks. This is my primary use case, I'm particularly keen to use DataClasses for readability.
• Adding shortcut values for readability, e.g. a simple plugin could decode "pi" -> np.pi.

Example Plugin:

I included an example plugin in test_pyon_plugin.py. The short is that you need to add
a new encode & decode function for each type that you want to add. They look like roughly:

@sipyco.hookimpl
def sipyco_pyon_encode(value, pretty=False, indent_level=0) -> str:
    if isinstance(value, CustomType):
        return pyon_encoding(value)

@sipyco.hookimpl
def sipyco_pyon_decoders() -> "Sequence[Tuple[str, Any]]":
    return [("CustomType", CustomType)]

Notes

This is not the cleanest implementation. I was aiming for minimal diff with the existing PYON infrastructure,
hence leaving in _Encoder class.
If there is a reason to extend other sipyco functionality, the pluggy infrastructure could handle that equally well.

I included an example & test for this, and all existing PYON tests continue to pass.

[drewrisinger]

Should probably change this to pm = plugin.get_plugin_manager(), then just add the register() at the end?