chore: add Aselo->Beacon integration stub Lambda code [CHI-3225]

[GPaoloni]

This PR is related to https://github.com/techmatters/infrastructure-config/pull/417.

Description

This PR:

• Adds stub Lambda code for Aselo->Beacon integration under lambdas/custom-integrations/uscr-dispatcher.
• Tweeks deploy Lambda action:
  • If a Lambda has a corresponding entry in .github/workflows/config/lambda-specific-region-map.json specifying which regions should it be deployed to, that's used.
  • Uses the default regions specified in .github/workflows/config/environment-region-map.json otherwise.

Checklist

• Corresponding issue has been opened
• New tests added
• Feature flags / configuration added

Verification steps

AFTER YOU MERGE

1. Cut a release tag using the GitHub workflow. Wait for it to complete and notify in the #aselo-deploys Slack channel.
2. Comment on the ticket with the release tag version AND any additional instructions required to configure an environment to test the changes.
3. Only then move the ticket into the QA column in JIRA

You are responsible for ensuring the above steps are completed. If you move a ticket into QA without advising what version to test, the QA team will assume the latest tag has the changes. If it does not, the following confusion is on you! :-P