Extracted logic into an npm package.

for your convenience, the main logic here around retrieving the installation access token has been extracted into a package call github-app-installation-token, in case you want to use this logic somewhere else.

this release uses that package under the hood, and now this only provides a thin, Action-related interface for that package.

initial publish of `github-app-installation-token-action`

this is the first release of github-app-installation-token-action! the v1 release marks the amount of features needed to facilitate the composition of GitHub Actions workflows using check_run events and GitHub Apps.