Support PRs from forked repos

[pradnyeshjoshi]

Description

Triggering execution of unit tests on AzureML requires access to credentials, which are not accessible from a forked repository, causing unit tests triggered by the PRs from outside collaborators to fail with a login error. This PR addresses the issue by using pull_request_target trigger, which allows execution of workflows in the context of a base repository, thus granting access to the AzureML credentials.
Here is a sample PR from a forked repo that can now successfully trigger AzureML tests #1746

Related Issues

Checklist:

• I have followed the contribution guidelines and code style for this project.
• I have added tests covering my contributions.
• I have updated the documentation accordingly.
• This PR is being made to staging branch and not to main branch.

[miguelgfierro]

@pradnyeshjoshi @simonzhaoms great job guys for finding a fix for this

[miguelgfierro]

@pradnyeshjoshi another question, do you need this PR to be in main to take effect? let me know and we can do a PR in that case

[pradnyeshjoshi]

@pradnyeshjoshi another question, do you need this PR to be in main to take effect? let me know and we can do a PR in that case

@miguelgfierro once we merge it into staging, people can make PRs into staging from their forks. Since we only make PRs to staging, no immediate need to release to main.