Implement download targets

This adds code to build a downloadable target's URL, download its
content, and write it to a local file.

Change-Id: Ie2c9238278ff735cc2f35c2ad08c10595fc3694f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/qg/+/126224
Reviewed-by: Erik Gilling <konkers@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
9 files changed
tree: 7b139602f57c9af32fa5e20ea369a0d2f026b323
  1. examples/
  2. qg/
  3. qg-cli/
  4. tools/
  5. .gitignore
  6. Cargo.toml
  7. rustfmt.toml