[FEATURE REQUEST]Alert Rule for external dependencies

### Problem Statement

It would be great to be able to create a alert rule for external dependencies. This would allow users of sentry that rely on external systems to monitor performance get alert when performance is degraded.

This also allows users to monitor based on performance based on a SLA that you have with a supplier.

### Solution Brainstorm

Filter on calls like: https://api.external-dependency.nl/*


### Product Area

Alerts