GitHub Actions shared composite actions

[zmughal]

Creates a set of composite actions for use across the organization. This is
based on the workflows in the https://github.com/PDLPorters/pdl repository.

Of note are the workarounds that use shell script to support conditional steps
by using if ${{ toJSON( <boolean expression> ) }}; then ...; fi since the
if: key is not yet supported per actions/runner#834.
This works because the output of toJSON() in that case is either true or
false which are the same as the shell script built-ins.

This does cause some issues as Windows does not work perfectly with shell: bash, but there are workarounds for that as well.

This is a first draft of this approach and there will be a set of associated
PRs applying this to different repositories.