[Pitch] Add CustomDebugDescription conformance to AnyKeyPath

[tshortli (Allan Shortlidge)]

@benpious

However, I started writing the SE and immediately ran into a new issue around API resilience: based on my reading of the rules, this change violates the requirement that existing types can't add a conformance to a protocol unless that conformance is added at the same time the protocol is added.

I believe this is no longer strictly the case now that conformances can be declared with availability. So the extension KeyPath: CustomDebugStringConvertible conformance would just need an appropriate @available(SwiftStdlib X.Y, *) attribute.