Add digests to download targets

This updates the definition of download targets within a manifest to
include a `digest` field, specifying what checksum should be calculated
on the downloaded file.

Change-Id: Ic3139f389ef47ab69c390b5b4729d6641e7970b8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/qg/+/126212
Commit-Queue: Alexei Frolov <frolv@google.com>
Reviewed-by: Erik Gilling <konkers@google.com>