Allow to amend the commit date and author date of any commit

**Is your feature request related to a problem? Please describe.**
I want to keep my history related to when I have done the work, but sometimes I need to commit a change after I have been working on it, so I need to amend the commit and/or author date of the commit.

**Describe the solution you'd like**
A suggest a new option under the amend menu

**Describe alternatives you've considered**
Making it manually, but the easy of use of lazygit will help.

I have a first version of a proposed implementation, but I think that it is better to create the PR after the feature has been evaluated.

https://github.com/japaz/lazygit/tree/amend-dates