[Feature Request] Support Asset Catalog compiler optimization

Xcode has a `ASSETCATALOG_COMPILER_OPTIMIZATION` build setting that:

> With no value, the compiler uses the default optimization. You can also specify `time` to optimize for speed of access or `space` to optimize for a smaller compiled asset catalogs.

I'd like to request to add an option specify the optimization value of `actool`. [Buck already does it](https://github.com/facebook/buck/pull/1016).

The issue does not name a file or test. Start by locating asset-catalog handling in rules_apple and the actool invocation, then trace how existing compiler options are exposed. Done means adding the requested optimization value and verifying the supported values in tests.

Written by the indexing model from the issue text.