Add API interface to read the value set of a variable within a variable library

Currently, it is possible to get the value sets for a variable library via Items - Get Variable Library Definition - REST API (VariableLibrary) | Microsoft Learn but since it's an LRO, there's a high cost in time & processing to get the values (e.g. wait for the operation, getting the defintion, procesing the body, converting the payload, etc.) 

The idea is to make avaialble an interface that returns the valueset for a given variable library/variable, example ... 

GET https://api.fabric.microsoft.com/v1/workspaces/{workspaceId}/VariableLibraries/{variableLibraryId}/variables/{variableId}/valueset

This will be equivalent to the functionality exposed by notebookutils 

samplevl = notebookutils.variableLibrary.getLibrary("sampleVL")
samplevl.test_int
samplevl.test_str