Add Independent Agent alternate step in pipeline

[dwhyrock]

This PR adds an alternate step in the elastic-stack-installers that the Independent Agent Release will use in order to get only the Elastic Agent windows installer (.msi) built.

The new pipeline step uses a new script that does the following:

• Retrieve the MSI artifacts from the Build step
• Moves the MSI artifacts to a new directory
• Creates a .sha512 file for the MSI file
• Saves the MSI and .sha512 artifacts using Buildkite's built-in upload functionality
• Sets metadata in the calling pipeline if the TRIGGER_JOB_ID is set
  • The calling pipeline will then use this metadata to download the saved artifacts

This PR incorporates the changes from these two PRs, which should be closed in favor of this PR:

• https://github.com/elastic/elastic-stack-installers/pull/285
• https://github.com/elastic/elastic-stack-installers/pull/281

I tested this PR with this unified-release PR, and verified that the Independent Agent Staging pipeline successfully triggers this new pipeline step, and is able to download the MSI and .sha512 artifacts after this triggered pipeline finishes running.

Example of successful execution:

• Ind Agent Staging pipeline: https://buildkite.com/elastic/independent-agent-release-staging/builds/271
• Triggered elastic-stack-installers pipeline for this PR triggered by the above pipeline: https://buildkite.com/elastic/elastic-stack-installers/builds/5635

[dwhyrock]

Closing in favor of a PR that is not from a fork: https://github.com/elastic/elastic-stack-installers/pull/287