Support multiple module descriptors with "mod add"

The CLI's mod add command currently relies on the working directory to determine the module descriptor for adding to Okapi. Like more recent mod commands, mod add could work with multiple module descriptors at a time. This can be accomplished by supporting input as a JSON array via stdin. By doing so, mod add could easily be piped data from mod descriptor which already supports the generation of module descriptors from an individual module, or an entire platform.

This enhancement would facilitate an additional step needed when provisioning a VM that doesn't' yet have descriptors for the current working platform (and are not yet available to pull from a remote registry).

This is related to STCLI-15 in that adding the platform modules should be included when not already present (or at leased checked for) with the all-in-one `platform backend` command.