feat: update `extra-files` based on the commit sha

**Is your feature request related to a problem? Please describe.**

While working on trying to update README file references after every version update, I thought that it'd be better if the `sha` was also updated along with the version, since this a really common pattern for GitHub Actions.

Another reason is automated updates (renovate, dependabot) where they most likely waiting for `sha` updates, rather than semver.

**Describe the solution you'd like**

Be able to get automated README updates using release-please, so the users of the workflow will now every time which is the latest version/sha of an action.

**Describe alternatives you've considered**

Create a custom action outside of release please to do this. While this may also be a viable solution, it was pretty obvious to me that going with `sha` updates would be something that other users would like as well to be included in release-please.