Muzzle should collect references from `additionalHelperClasses` during codegen

**Is your feature request related to a problem? Please describe.**
Currently we don't collect references to classes defined in the `InstrumentationModule#getAdditionalHelperClasses()` method (nor do we collect references made by these classes' contents). 

**Additional context**
https://github.com/open-telemetry/opentelemetry-java-instrumentation/pull/4866#discussion_r767483772
