cli: Add support for new runtime entity whitelist admission policy format

[abukosek]

Recent changes for ADR 0004 have modified the runtimes' entity whitelist admission policy structure, so that it's possible to specify the maximum number of nodes that an entity can register under the given runtime for a specific role.

This requires the command-line parsing in go/oasis-node/cmd/registry/runtime/runtime.go to be updated.

In the current implementation, the same syntax as before configures an unlimited number of nodes of any role for the given entry, which matches previous behavior.

Parsing a nested map on the command-line might not be the best way to solve this. Perhaps a configuration file would be more user-friendly.