Option to only show datasets that are available in the viewport

[cholmes]

Right now there's a number of datasets that aren't global, but you just have to guess that from their name. And even global datasets don't often have coverage everywhere, like most don't cover the ocean, but you don't know that till you download.

For the first just checking the bounds of the datasets (perhaps store in the plugin? Ideally stac or something) would work. For the second it seems like you'd need some raster mask of availability. Could be interesting to explore that as part of STAC standardization - a little asset that is just a single bit raster of presence / absence like per degree cell or something...

#17 is a bit related