Add caching for imported assets

[realkotob]

This enables caching for imported assets between each run. This massively speeds up the export stage for big projects (which needs ~30 minutes to import 150MB for a 3D project).

Note that this saves the cache after a stage is done therefore new imports are not shared by the 3 platforms if they run in parallel. So if you add new files it will still import them once for each platform (but will not import them again in the future, which is what matters). There is no way to optimize this further at the moment since there is no separate CLI command to reimportat assets.

[realkotob]

Relevant docs:

• https://docs.gitlab.com/ee/ci/caching/
• https://docs.gitlab.com/ee/ci/yaml/README.html#cache

[abarichello]

Thanks for the contribution!