Add deployer test pipeline

[hamza-itential]

Added a GitHub Actions workflow to test the deployer by running it on a set of EC2 instances. Runs upon creating a PR to merge to the main branch.

Notes:

• There are currently 4 test cases: one for each of the 4 validated designs (aio, minimal, ha2, asa). Each currently uses the same EC2 template (Rocky 9 - the Themis default).
• The pipeline currently uses extra vars to override the behavior in the Themis var files to install Redis from the Remi repo using a specific repo RPM URL. This is as a temporary workaround until the relevant issues in the deployer (dependency resolution error while building from source, incorrect Remi repo URL) can be resolved.