Support loading XML assets (#45334)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/45334

Adds support for the `xml` file extension as a loadable asset, and lets Flow treat the type signature as an image

Changelog: [Internal]

Reviewed By: robhogan

Differential Revision: D58261501

fbshipit-source-id: b29cffee81d0438827529711e86267edd5d2a0f7