Internal: Implement "high risk changes" PR alert

[kahest]

Description

Implement a mechanism that automatically adds an "alert" comment to PRs if they include changes to files we think have higher potential to break things than others.

On sentry-cocoa, we use the following:

• we define a list of high-risk files
• we run a check that adds alert comments to PRs if they change a file that's part of above list