Add basic support for type hinting-based docs

[viniciusd]

Incrementally addresses #128

A working example that fully works after these changes:

from sanic import Sanic
from sanic_openapi import doc, swagger_blueprint

app = Sanic()
app.blueprint(swagger_blueprint)

class Car:
    make: str
    model: str
    year: int

@app.get("/car/<id:int>")
@doc.summary("Gets a car")
@doc.produces(Car)
async def get_car(request, car_id):
    return json({
                 "make": "Nissan",
                 "model": "370Z",
    })

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8000)

[viniciusd]

I acknowledge there aren't tests related to this kind of feature throughout the current codebase. In a future PR, I can work on adding tests to the object serialization.

[chenjr0719]

@viniciusd Sorry for the delay. About the conflict, please solve it. Maybe you can put the example at here. And, I think it is better to add test in this PR or the code coverage might decrease after merge this PR.

[codecov-io]

Codecov Report

Merging #129 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
+ Coverage   95.54%   95.56%   +0.01%     
==========================================
  Files           4        4              
  Lines         449      451       +2     
  Branches       95       95              
==========================================
+ Hits          429      431       +2     
  Misses          6        6              
  Partials       14       14

Impacted Files	Coverage Δ
sanic_openapi/doc.py	100% <100%> (ø)	⬆️

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[viniciusd]

Thanks for the feedback!
Conflict is now solved 👍

Gonna add the example to the docs

[viniciusd]

Sorry for the delay, got pushed back by a few other tasks.

Added docs to the fields.md file :)

[chenjr0719]

Nice work.