PIN 16: Implement `Result` subtype mimicking `ConstantResultHandler`

[lauralorenz]

Use Case

See PIN 16 (PR #2134) for background. This is a spin-off from #2141 for only one (special) handler, ConstantResultHandler, which has internal implications.

Solution

Implement a Result subclass for ConstantResultHandler that knows how to .read, write, and check if it .exists and operates properly for the same internal purposes as the current ConstantResultHandler does.

Alternatives

Please describe any alternatives you've considered, even if you've dismissed them

[lauralorenz]

Not sure why I can't tag @alexisprince1994 as assigned to this issue (as they opened a PR); assigning to myself for now just so it's not confusing on the board.

[joshmeek]

@lauralorenz Hmm we must only be able to assign those in the Prefect org 🤔