Expose amp_cache_transform lib in transform_wasm

This could be useful for consumers of the wasm lib who are using it to build a signed exchange generator.

Start by locating the transform_wasm library and the amp_cache_transform code or entry point. Determine how the wasm library exposes functionality to consumers building signed exchange generators; done means amp_cache_transform is available through transform_wasm and the relevant existing checks pass.

Written by the indexing model from the issue text.