Create tar_terra_svc() for SpatVectorCollection objects

[Aariq]

I think at a minimum we should add an analog for SpatVectorCollection (i.e. tar_terra_svc() for terra::svc() results)

My initial response in the case of lists, which can be nested and heterogeneous, is that this may be a bit difficult to handle in general for geotargets methods.

However I can totally see the utility of list columns within a data.frame being a useful way to manage multiple terra objects and their metadata... so this is perhaps worth considering more.

Originally posted by @brownag in #77 (comment)

[Aariq]

This will be more difficult to implement because:

1. the equivalent to sub-datasets appears to be layers, which seem to be only supported by a limited number of formats (see insert arg in ?writeVector)

2. When sds() is used on a file with multiple layers (saved using the insert arg), it can only read a single layer apparently (???)

[Aariq]

The reason I'm looking at this is because I'm thinking of possibly converting tar_terra_sprc(), tar_terra_sds(), and tar_terra_svc() into something like tar_terra_collection(name, command, type = c("sprc", "sds", "svc").

[Aariq]

This seems tricky to implement so I think I'll hold off on this until after rOpenSci submission or if there's some user need for it