Configurable embark-keymap-help pseudo-action

[oantolin]

btw, another thing i'd like to have is something similar to the embark-keymap-help pseudo-command, but to invoke my embark indicator on demand, as it's done now with embark-completing-read-prompter after C-h... i would then set embark-indicator to its default, and then hook the invocation of my own indicator to, say, C-h: that would invoke my indicator and stay in embark-keymap-prompter, so that i don't need to replicate the latter's logic in a full new prompter.

to put it another way, i'd like to be able to customize the function called when the prompt receives the command embark-keymap-help. but i can always :override embark-completing-read-prompter with the one i want, so maybe this is not worth complicating embark proper.

Originally posted by @jaor in https://github.com/oantolin/embark/issues/132#issuecomment-767795737

[minad]

Seems to be the same issue as #319?