ActionPrompts: More flexibility for selection_command

[FHeilmann]

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What does this PR do and why is it necessary?

I'm currently playing with klipper and octoprints action prompts and ran into the following issue:
I wanted to have a custom selection_command in the shape of HANDLE_PROMPT_RETURN CHOICE={choice} to call one of my klipper macros. Unfortunately, Octoprint simply appends S{choice} to whatever you put into the settings/config.yaml, so my command would end up looking like this:

HANDLE_PROMPT_RETURN CHOICE={choice} S0

This PR changes how the action prompt plugin forms the command, by checking if the user put {choice} into the respective configuration himself. If that is the case, it simply replaces {choice} with the choice. Otherwise, it behaves as it previously has.

How was it tested? How can it be tested by the reviewer?

Tested with both M867 and HANDLE_PROMPT_RETURN CHOICE={choice} as value for selection_command . Both variants yield the expected result

I didn't want to fiddle with my octoprint pi too bad so I jerry-rigged it into the Octopi install (which may give me more trouble in the future I admit). Therefore I have not run the test suite against this.

Hope this PR is appropriate
-Florian

[foosel]

👍