Support for selecting region in custom dashboards

[bcoe]

Problem Statement

We've been adding support for region selection to insights pages, see: #75230

It would be great to also have this functionality when filtering custom dashboards (this feature request originally came from a Sentry user who's a heavy custom dashboard user).

Solution Brainstorm

Add the same regional selector available on Web Vitals / Mobile Screens to Dashoards.

Product Area

Dashboards

[ale-cota]

Closing this in favour of the more generic custom filters #55036