gnmi-3.1 union_replace origin README and test Code

[dplore]

Create README and Implement test code for gnmi union_replace which validates the ygnmi functions for BatchUnionReplace and BatchUnionReplaceCLI for some very basic OC and CLI combinations. This will reside in the feature/gnmi folder as a base test of gnmi functionality.

Deeper testing for mixing OC and CLI are expected to be covered within the feature test being tested. For example BGP tests which require deviations mixing CLI and OC together should be testing using union_replace and placed in the feature/bgp folder.