Add `exec` to rebase interactive

**Is your feature request related to a problem? Please describe.**
Sometimes i need to do a change in some past commit (or in several commits) during an interactive rebase With `git rebase --interactive` i have the oportunity to insert some `exec` commands between picks, squashes, etc. so those changes are applied

**Describe the solution you'd like**
Being able to add press a key, say `x` and insert a command and `exec` it on the selected commits similar to the interactive git rebase

**Describe alternatives you've considered**
Maybe this feature could be implemented as a custom shortcut, but i don't have enough knowledge about lazygit shortcuts customization to try it myself