chore(ci): add a separate PR workflow for forks

[nirinchev]

Description

I was a bit annoyed that the dependabot workflows never run due to secrets missing. This adds a pull_request_target workflow which only runs for forks and dependabot PRs.

And since I was also annoyed by the constant warnings, I went ahead and added the return types for all functions.

Checklist

• New tests and/or benchmarks are included
• Documentation is changed or added
• I have signed the MongoDB Contributor License Agreement (https://www.mongodb.com/legal/contributor-agreement)

Motivation and Context

• Bugfix
• New feature
• Dependency update
• Misc

Open Questions

Dependents

Types of changes

• Backport Needed
• Patch (non-breaking change which fixes an issue)
• Minor (non-breaking change which adds functionality)
• Major (fix or feature that would cause existing functionality to change)

[nirinchev]

I'm going to go ahead and merge this to test it out, but feel free to leave a review and I'll address any comments in a follow-up.