Add z/OSMF system variables GET API

[AditheD]

Summary

• Add VariableGet API for retrieving z/OSMF system variables
• Add VariableGetResponse and VariableResponse response models
• Add unit tests for VariableGet and response classes

Fixes #505

[frankgiordano]

IBM documentation says you can have two different urls for this REST API:

GET /zosmf/variables/rest/<version>/systems/<sysplex-name>.<system-name>

and

GET /zosmf/variables/rest/<version>/systems/local

you are only covering the first case. You need to implement the second one too.

I suggest you create

public VariableGetResponse get(final String sysplexName, final String systemName)

public VariableGetResponse getLocal()

private VariableGetResponse getCommon(final String sysplexName, final String systemName, boolean isLocal)

in here do a if check and construct the url based on the boolean value.

[frankgiordano]

At this point, this is too many parameters to keep track of for the end user and for our code logic.

Let's do a factory pattern.

See https://github.com/zowe/zowe-client-java-sdk/tree/main/src/main/java/zowe/client/sdk/zosfiles/uss/input/factory

Create VariableGetInputFactory.java and VariableGetInputData.java under a factory package.

Then remove all the get methods and only leave one called get(VariableGetInputData inputData)