Add Option for Specifying Different Metadata and Targets URLs

[justdan96]

Currently there is no way to specify different metadata and target URLs for the same repository, this means that trdl can't handle repositories such as the Bottlerocket one. For reference this is how the same is handled in tuftool:

VERSION="v1.6.1"
VARIANT="vmware-k8s-1.24"
OVA="bottlerocket-${VARIANT}-x86_64-${VERSION}.ova"
OUTDIR="${VARIANT}-${VERSION}"

tuftool download "${OUTDIR}" --target-name "${OVA}" \
   --root ./root.json \
   --metadata-url "https://updates.bottlerocket.aws/2020-07-07/${VARIANT}/x86_64/" \
   --targets-url "https://updates.bottlerocket.aws/targets/"