Custom code generation from `serde-reflection` metadata

[charypar]

Current codegen for Swift, Kotlin and TypeScript relies on serde-generate. This works quite well, but has a few limitations/challenges:

1. Generics support across the FFI boundary (especially for Option<T> and Result<T>) - mostly because discovering a generic type with serde is non-trivial and probably not exactly reliable
2. Generated TypeScript code handling of enums could be improved on, especially enums could be more idiomatic
3. Adding support for other shell languages is challenging. The obvious example is Dart in order to support Flutter

[charypar]

#206 is another example of current typegen causing problems

[charypar]

We should take a second look at https://github.com/1Password/typeshare