fetchSoilGrids: add grid argument for downloading COG subsets for extent

[brownag]

Will close #202

As complement to point-based query already provided, provides an option to download the source grids for arbitrary extent.

For example:

library(terra)
library(soilDB)

x <- fetchSDA_spatial("CA630", by.col =  "areasymbol", geom.src = "sapolygon")
#> Using 1 chunks...
#> Chunk #1 completed (n = 1; 0.9 secs)
#> Done in 1.5 secs; mean/chunk: 0.9 secs; mean/symbol: 1.54 secs.
x <- crop(vect(x), ext(x) / 6)

res <- fetchSoilGrids(
  x,
  grid = TRUE,
  filename = "sg_test.tif",
  depth_intervals = "5-15",
  variables = c("bdod", "cec", "nitrogen", "cfvo"),
  summary_type = "Q0.5",
  verbose = TRUE
)
#> 0...10...20...30...40...50...60...70...80...90...100 - done.
#> 0...10...20...30...40...50...60...70...80...90...100 - done.
#> 0...10...20...30...40...50...60...70...80...90...100 - done.
#> 0...10...20...30...40...50...60...70...80...90...100 - done.

plot(res)