Mode-specific hydra commands (a. k. a. read-extended-command-predicate compatibility)

[kenranunderscore]

I'd like to have the option to make the commands created by defhydra mode-specific. That is, I'd like them to be available in M-x completion only in the modes they're defined for.
This is what read-extended-command-predicate was introduced for in Emacs 28.x, and the way to do it is to specify these modes in the call to interactive.

Is this something you'd consider adding? I haven't checked the code base yet, but if it's not too hard, I could take a stab at it; but I'd need to know what the public API should look like. (For comparison, here's where I added this change to reformatter.el: purcell/emacs-reformatter#61.)