LibraryVariableSet Import and Export

When importing and exporting a project, the project-level variables are included. However, if the project references a library variable set, there is no command to import and export the variables and values in the shared set. This changes adds an importer and exporter of type "libraryvariableset" to handle these shared variable values.

The underlying VariableSet that stores the values is the same for LibraryVariableSet as for Project, so most of the code for import and export is copied from ProjectImporter. As a result, there is a lot of duplication in the code, especially in the more complicated import logic. However, I wanted to keep it simple for this initial request and avoid modifying any existing code.

I have an additional [variable-import-refactor](https://github.com/kpitt/Octopus-Tools/commits/variable-import-refactor) branch that builds on this change by refactoring ProjectImporter and LibraryVariableSetImporter to share the common code.