Enable cache option

Since caching is still likely unused by anyone (the new descriptors won't be merged into Bitcoin master until after the next stable release), add a switch to enable caching explicitly.

This way projects that may mix builds with/without the cache won't have to worry about cached files being uploaded/downloaded when they're not actually being used.

Real-world example:
Soon, new Bitcoin descriptors with cache enabled are merged.
Soon-after, a maint release of Bitcoin requires that an old branch be built without the cache.
Those builds will upload/download the cache without using it, which is confusing and could be time-consuming.

Instead, the functionality will only be enabled for descriptors that use:
```
enable_cache: true
```