Support Uptime Monitor Widgets on Dashboards

### Problem Statement

Currently, Uptime Monitor data cannot be added to Sentry Dashboards. This limits teams, especially non-engineering teams like customer support—from viewing overall app health alongside other key metrics.

### Solution Brainstorm

- Add Uptime Monitors as a supported widget type.
- Allow basic uptime metrics (e.g. availability, incidents) to be displayed.
- Enable customization similar to other widgets (filter by tag, environment, etc.).

### Product Area

Dashboards

The issue names no files, tests, or entry points. Start by locating the dashboard widget and Uptime Monitor implementations, then clarify the supported metrics, filters, customization behavior, and acceptance criteria before making changes.

Written by the indexing model from the issue text.