support generated code customization

[zerlok]

User may wish to tune the code (optimization or better readability / documentation). Thus, simple codegen which overwrites exiting modules becomes useless.

Case:

1. user generates code first time (invokes gendalf cast)
2. user customizes the generated code (some methods / DTO field types)
3. user generates code second time -- gendalf modifies only own code, doesn't change user's code

Consider which changes gendalf should avoid and is it useful.

• gendalf should change a set of fields in DTO and mapping
  • allow & keep custom validators on existing fields (e.g. email: str , but user wants to validate it with email regexp)
• gendalf should change server & client signatures
• gendalf should not change server & client existing method implementations. Mypy will complain on types mismatch.

Also, add force flag to overwrite whole module. Consider entrypoint / method filters for this flag.