feat(cli): add option for downloading all models, refine help messages (#1061)

* chore(cli): update download help messages

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

* add `--all` flag to model download CLI

Signed-off-by: Panos Vagenas <pva@zurich.ibm.com>

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Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>
Signed-off-by: Panos Vagenas <pva@zurich.ibm.com>