Bulk invite for users -> namespace and namespaces -> package

> cli/lib/commands/invite.js
> I do note that it would be nice to invite a list of people at once.

When creating a registry or restructuring a team, namespace members/package maintainers will likely change in large quantities at a time, so it will be very handy to have endpoints capable of handling bulk payloads. The CLI should accept either a list of parameters or a path to a file containing a list of inputs.

## Affected commands

- `ds invite`

## Related

Depends on #188.