Release automation for Shadow.

[leegeth]

Release automation for Shadow.

Description of changes:
Add a GitHub Action job to trigger a release process which includes operations of:

1. Creating and pushing the release tag to the repository
2. Verifying the pushed tag (performing a diff with the commit ID which is tagged)
3. Creating a ZIP for the release asset.
4. Verifying the ZIP by performing a diff check and running unit tests on the unzipped files. 5. Creating a release on the repository for the tag, and uploading the ZIP as the release asset

The workflow can be manually triggered and takes the input values of Commit ID (to create a release for) and the Version string for tagging the release with

Test run of the GHA run on my fork
https://github.com/leegeth/Device-Shadow-for-AWS-IoT-embedded-sdk/actions/runs/406525459

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.