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

[dsotirakis]

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.