CLI: Add functionality to operate on Variables

[rajdip-b]

Description

We would like to perform the following operations for variable using our CLI:

• List all variables under a project (keyshade variable list <projectId>)
• Fetch all revisions of a variable (keyshade variable revisions <variableId>)
• Create a variable (keyshade variable create <projectId>(refer postman for parameters))
• Update a variable (keyshade variable update <variableId> (refer postman for parameters))
• Rollback a variable (keyshade variable rollback <variableId> --environment-slug/-e <> --version/-v <>)
• Delete a variable (keyshade variable delete <variableId>)

Solution

• Create an implementation of command.interface.ts and name if VariableCommand.
• Stash all the functions in src/commands/variable.
• Use the variableController from ControllerInstance to make the API calls.