Cache extension build results

[britzl]

To speed up CI builds (which are usually run from a clean environment) where extensions rarely change it will help if extension outputs (library+resources) are cached on the extender server.

[britzl]

Add metrics first to measure before and after this change.

This will reduce build time. The downside is that it will increase storage requirements (and by extension costs).

[JCash]

Metrics ticket: https://github.com/defold/extender/issues/289

[JCash]

Perhaps do it in these steps:

1. full upload
2. extensions
3. libraries
4. jars

[AGulev]

Also, it makes sense to take into account situations when the only change is changes in manifests. For example, CI builds game and increase version number in game.project. This version number ends up in AndroidManifest.xml. And this is the only change from the previous build.