✨ Feature Request

Enable optional file-formats, provided as extension packages, to modify the Iris process of "identify a file type and invoke the correct loader"

Motivation

So Iris can load unstructured data automatically, if iris-ugrid is available, as it does for iris-grib.
But also, because the "io picker" code still includes (minimal) support code for optional formats (GRID, and to-be UGRID), which should really be coded entirely in the extension packages.

Additional context

Grib-specific support remains in core Iris here
We would like to tidy that, and to add UGRID support in a similar way, so we need to fix that.

Notes :

1. we also need to invent some way of controlling relative picker priorities, which can work when we no longer have a simple static list of available format handlers.
2. 'ugrid' support needs to overlay the standard netcdf file-pickers. When no mesh is found in the file, this should behave exactly as it did before.