Add a SPEC_PROCESSOR_PASS_OBJECT to control the argument type of the spec processor. Example usage:

from apiflask import APIFlask

app = APIFlask(__name__)
app.config['SPEC_PROCESSOR_PASS_OBJECT'] = True

class FooSchema(Schema):
    id = Integer()

@app.spec_processor
def update_spec(spec):
    spec.title = 'Updated Title'
    spec.components.schema('Foo', schema=FooSchema)  # add a schema manually
    return spec

fixes #213

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue.
• Add *Version changed* or *Version added* note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.