Option to use system temporary directory

[dlid]

As I suggested in #218 here's a PR for adding an option to use system temporary directory instead of the output directory.

I have noticed a large VSCode workspace may use a lot of CPU when a lot of files are created/deleted and VSCode wants to index the files for its search.

The benefit of this would be that the tempDir and all its files would not affect the workspace or repository for the code that other applications may use and react to. As earlier, only the affected files will be copied/deleted from the final output directory so the output directory changes would be limited to the changes that ng-openapi-gen detects and copies to the final output directory.

• The option useTempDir was added to schema and options.ts
• The function initTempDir in ng-openapi-gen.ts to use system temp directory if option is specified
• Two unit tests was added to test tempDir path with and without the option specified