An option in kitspace.yaml for checking a specific SHA

[AbdulrhmnGhanem]

In #526 (comment), I proposed using forked repos for registry borads. But after thinking about it I think there is a better solution.

The forked repos approach is problematic because we have to be alerted whenever a user makes a breaking change in the repo, then we have to fork this repo and maintain it which won't scale well when the numbers of repos increases.

Instead, we can introduce an option to in kitspace.yaml to pin the project a specific SHA if this option isn't used we will use the HEAD of the repo as usual. We can open PRs in the repos in the registry to add this option so we won't need to maintain a registry file for current and future repos.