xpath: adds support for prefixes in list keys

[sgg]

This diff adds support for prefixes in list keys as well as some unit tests.

Context

Where an identifier is something like interfaces, an identifier with a prefix might be openconfig-interfaces:interface. The ABNF in the YANG RFCs refers to these as node-identifier rules.

The current xpath implementation does not allow prefixed identifiers in list keys, however per RFC 6020 and RFC 7950 these are valid.

We can look to openconfig for a real world example.

For completeness I've lifted some relevant grammar snippets from the YANG RFCs:

   identifier          = (ALPHA / "_")
                         *(ALPHA / DIGIT / "_" / "-" / ".")

   prefix              = identifier

   node-identifier     = [prefix ":"] identifier

   absolute-path       = 1*("/" (node-identifier *path-predicate))

   relative-path       = 1*(".." "/") descendant-path

   descendant-path     = node-identifier
                         [*path-predicate absolute-path]

   key-arg             = node-identifier *(sep node-identifier)

   key-arg-str         = < a string that matches the rule
                           key-arg >

   key-stmt            = key-keyword sep key-arg-str stmtend