Force read into memory

[Aariq]

This is a necessary addition to tar_terra_rast() for SpatRaster targets to work with cloud storage. (see more discussion in ropensci/targets#1367). I added a parameter and some documentation, but haven't figured out how to "hook it up" efficiently. When force_memory is true, it needs to modify the body of the read function to add + 0 or * 1 to the end. Tried messing around with body(fun) and rlang::expr() but didn't get anything satisfactory. I'm open to ideas and alternatives!

One alternative is to use the new content addressable storage tools in targets to have a custom download function that doesn't actually download the target, but rather returns a /vsis3/ filepath to the read function. This is not something we'd implement in geotargets, but would be something cool to document in a vignette. I may play around with this in the next few weeks.

[Aariq]

I actually think documenting the content addressable storage workaround is probably a better way to go, but I'll leave this open for a bit just in case y'all think there might be good reasons to add a force_memory argument.