Metric alerts should have an option to add an external URL

### Problem Statement

Often, when an alert is triggered, it may be crucial for teams to access additional information or documentation related to the issue immediately. This might include links to Confluence pages, incident reports, documentation, or other external resources.

Currently, there is a limitation regarding the ability to provide additional context or resources when an alert is triggered.

It would be helpful if users had the ability to add external URLs to alerts through a text box in Sentry. By having the ability to add external URLs to alerts, incident response processes can be streamlined providing teams with easy access to relevant information.

### Solution Brainstorm

External URL Field: Introduce a new field in the alert creation/edit interface where users can input an external URL. This URL could point to relevant documentation, runbooks, or any external resource that provides context about the alert.

URL Visibility: Make the external URL visible in the alert details or notification message so that when a user is notified through channels like Slack or email, they can see and access the URL directly.

Customization: Allow users to customize the label or name associated with the external URL so that it's clear and descriptive.

### Product Area

Alerts