Add custom GH action for installing and configuring Spread

[cjdcordeiro]

Given that Spread is being used in multiple GitHub repositories to assist with the testing of software, this PR introduces a custom composite GitHub action to simplify and streamline the installation and configuration of the Spread binary on the GitHub runners.

What's included:

• a new action.yml file where the composite GH action is defined. It accepts the Spread repo branch/tag/sha to be checked out (default is master). It also outputs the final path of the spread binary on the GH runner.
• an additional step to the existing test workflow, to make sure the new action behaves well
• an update to the README.md file to document this new GitHub action

Example of a successful run: https://github.com/cjdcordeiro/spread/actions/runs/5879643153/job/15944157818