Add a "Custom Widget" option — widgets are locked one-per-category, can't mix data source and visualization

[Kirubananthan-Kuppan]

Describe the problem
Creating a dashboard has a "Blank Dashboard" option for full custom freedom.
But inside widgets, there's no equivalent "Blank Widget" — every widget is
locked under its own category: MonitorList under Monitors, Chart under
Metrics, etc. You can't, for example, pull Monitor data into the Chart
visualization — each category only works with its own data source.

What I want
A "Custom Widget" option where I can independently choose:

1. Data Source — any of Monitors, Metrics, Logs, Incidents, Alerts, Security
   Events, Traces, SLOs, or External Data Source
2. Visualization — any type (Chart, Table, Gauge, Stat, Bar, Pie, State
   Timeline, Heatmap, etc.), not restricted by data source
3. Configuration/Settings — fields, filters, Group By, colors, tooltips —
   fully user-defined

Screenshots

Desktop:
Version: OneUptime 12.0.28
Deployment Type: Self hosted