Better APISpec init to allow to modify it before generating spec_json.

[ddorian]

This will make it easier for APISpec to be modified by external functions before generating the spec_json (examples: adding webhooks, registering custom components etc).

Checklist:

• Run pytest tests and no failed.
• Run ruff check flask_openapi3 tests examples and no failed.
• Run mypy flask_openapi3 and no failed.
• Run mkdocs serve and no failed.

close #190

[luolingchun]

This PR gave me some inspiration.

I don't quite agree with PR #190's approach, self._register_default_error_responses() only refactored part of the functionality.

You can expose self.generate_spec_json() function on top of this PR to implement spec_json refactoring.

self.generate_spec_json() contains all the remaining logic

@property
def api_doc(self) -> Dict:
    if self.spec_json:
        return self.spec_json

+   self.spec_json = self.generate_spec_json()
+   return self.spec_json

[ddorian]

The difference is if you want to modify the whole thing or piece by piece.

Both ways would be best IMO.