macros-core: Refactor service code generation

[DennisInSky]

File Location(s)

macros/core/src/service/mod.rs

Proposal

Make code generation for the gservice macro more modular:

• patch service
• generate exposure
• generate meta
  etc.
  It may require some extra passes over the input, but we can sacrifice it in favor of code maintainability and readability
  Think of making use of genco if code generation is simpler than using proc_macro::quote