Add a possibility to run a script before v2-sdist

[phadej]

That script could be used to automatically run autoreconf -i, or something more fancy like in #6180

The main motivation is to ease the conflict between not checking in autogenerated files, and allowing to use source-repository-package (which doesn't sdist now, but could). We need a way to do some preparation tasks. Both autoreconf -i and generating VCS manifest can be done in such way that cabal get package and then cabal v2-sdist would still produce the same file.

[phadej]

there is now.

[hasufell]

there is now.

where?

[phadej]

there is now.

where?

https://cabal.readthedocs.io/en/3.6/cabal-project.html?highlight=%22source-repository-package%22#cfg-field-post-checkout-command