Allow local artifacts storage to be used for dependencies

[petrows]

We have issues that Github might return 500 on some of downloads (i.e. terraform tool) and that causes annoying CI failures.

Please add option to specify tooling deps in kobra.yaml, for example:

# kobra.yaml
toolchain:
  download_prefix: https://my.files/storage/kobra/
  tf:
    provider: terraform
    version: 1.2.3
  ansible:
    version: 1.2.3.0
  sops:
    version: 1.2.3

This should point Kobra to something a-là https://my.files/storage/kobra/terraform-1.2.3.tar.gz.