`extra-executables` in cabal.project

**Describe the feature request**
A new `extra-executables` field that specifies executables to be provided during development, such as linters and formatters. These should be solved independently from each other and from the main project.

The field should **not** contain executables that are needed to _build_ the project. Those go in `build-tool-depends`, in the appropriate .cabal file

**Additional context**
Originally #7865 and https://github.com/haskell/cabal/issues/6952#issuecomment-971440004