feat(api): Allow omitting description and display_color from ProtocolContext.define_liquid()

[SyntaxColoring]

Overview

Closes AUTH-633. This is pretty simple and self-contained, so I figured we might as well try to squeeze it in v8.0.

Test Plan and Hands on Testing

Play with different apiLevels for this "protocol":

requirements = {"apiLevel": "2.20"}

def run(protocol):
    protocol.define_liquid(name="Liquid name", description=None)  # display_color omitted.

Changelog

• When define_liquid() is called without supplying any value for description or display_color, act as if None was supplied. This takes a little bit of trickery to retain the existing behavior for older apiLevels, which is to require some explicit value, even if that value is just None. The new behavior is gated behind apiLevel 2.20.
• A little bit of Sphinx trickery to prevent the trickery above from making the method signature look really confusing in the reference documentation.

Review requests

Do we want any of these docstring changes in a different branch?

Risk assessment

Lo.

[sfoster1]

Yes, that looks very good to me. Love a sentinel.

[ecormany]

I could nitpick on the length of the versionchanged description, but everything in it is accurate.