CLOUDP-304211: Only accept pull_request_target with safe-to-test label

[josvazg]

This PR uses a new small trigger.yml workflow to handle the logic for both pull_request & pull_request_target separately of other ways to trigger the test.yml workflow. There are only 2 scenarios allowed:

1. The pull_request events without the safe-to-test label, intended for code owner contributions. Uses the incoming change repo creds, which is the same, as the PR comes from a branch in the same repo.
2. The pull_request_target events with safe-to-test label that a code owner have just set. Uses the target repo creds, that is the AKO repo creds, and is intended to allow contributions to be tested once a code owner decides it is safe to do so.

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[josvazg]

The only way to activate this change and test it is by merging it first, unfortunately. This is because the pull_request_target we need to test runs on the base branch (main) context, and will not have the new trigger logic until it is merged in.

[josvazg]

CLOUDP-304211