Add support for parsing wildcard instance identifiers to JSON codec

While https://tools.ietf.org/html/rfc6020#section-9.13 requires instance identifiers to have keys,
it would be useful to be able to parse unkeyed ids to YII and then II (both support wildcards).

Simplest solution would be to make JSONStringInstanceIdentifierCodec accept
wildcard instance identfiers.

Example use case would be to define processing order for yang nodes in human friendly format, e.g.:

/ietf-interfaces:interfaces/interface -> /v3po:bridge-domains/bridge-domain