feat(get-merge-base): add get-merge-base helper

[sayali10]

📝 Description

• Add get merge base for pull request.

API Used

• https://octokit.github.io/rest.js/v20#pulls
• https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#get-a-pull-request

UseCase

• Want to use mergeBase to get specific diffs for files.
  e.g. We want to determine, which dependencies are updated in package.json file, to get that info, we want to get mergeBase for PR and use it to calculate diff for package.json devDeps using mergeBase and PR head.

[danadajian]

Could you use ${{ github.event.pull_request.base.sha }} in the workflow context to get this?

[sayali10]

Able to get it SHA using github.event.pull_request.base.sha. This change is no longer required.