Support specifying the LeanInk path

[utensil]

LeanInk is sensitive to Lean versions and expected to be installed locally, it's useful to support specifying the LeanInk path via a flag like --leanink= in such scenarios.

A similar flag (but as --ink) is present in https://github.com/leanprover/doc-gen4 to invoke LeanInk.

For more details, see the discussion here: leanprover/LeanInk#21 (comment) .

For implementation, it seems that somehow reusing binpath of CLIDriver could make it work, but it's not so easy to pass it to the driver in the current generic way to resolve the CLI driver.

[cpitclaudel]

That would be nice, and same for the driver path of the other drivers. Maybe we could add a --driver-executable flag or something similar?