Add PathElemsMatchQuery helper for checking path membership to a telemetry path

[wenovus]

This helper checks whether a particular concrete path matches a query path per gNMI path specifications

NOTE: the new pathstrings.go is COPIED from the ygot package, as otherwise we have a circular dependency. In the future, the copied helpers should move to the util package.

[coveralls]

Coverage increased (+0.02%) to 90.634% when pulling 78691dba0669247fa59cfdd3382a06e9bffbb5ee on wildcard-path-helper into 00a6fee629af5993c9974511b3bcb5c321024cf1 on master.

[robshakir]

Wen, this was on the review stack but looks to have outstanding comments. Is there anything you need from me here?

[wenovus]

It's actually not this PR, but rather this comment: https://github.com/openconfig/ygot/pull/467#discussion_r707869638 I'm wondering if you think it's worth splitting the util package up into these packages in order to make the functionalities more modular. If you think this is a good idea we might consider doing this prior to v1 release.