[FR] Customize metadata generation to avoid adding unnecessary unused JS wrappers

[mbektchiev]

Is your feature request related to a problem? Please describe.
Currently, the whole iOS SDK and all native libraries used in a {N} app are included in the metadata binary files. In order to reduce app size and improve startup performance, we should allow app/plugin developers to specify which entities (e.g. clang modules) need to actually be accessible from JavaScript.

Additional context
Things to consider:

• This will be a breaking change because every plugin and every app which directly call into native code will need to start stating their native dependencies
• Multiple such dependency lists coming from all plugins and the app should be merged. Maybe some changes in the CLI will be necessary as well.
• There should be a way to allow everything to be included (like it's been before this feature)

An idea for UI which helps generate these lists from @shirakaba: Polyfill.io shows one user interface by which one could generate a package by specifying only the libraries that one needs. Maybe something like this could be applied to the NativeScript runtimes: https://polyfill.io/v3/url-builder/

Corresponding feature request in Android runtime is: NativeScript/android#1485

Implemented in {N} CLI with: NativeScript/nativescript-cli#5220