support schema union in query

[chang-ph]

Allows query parameters to be defined as the union of schemas. For example:

class Rect(Schema):
    type: Literal["RECT"]
    width: float
    height: float

class Circle(Schema):
    type: Literal["CIRCLE"]
    redius: float
    x_center: float
    y_center: float

class QueryParams(Schema):
    q: Annotated[Rect | Circle, Field(discriminator="type")]

@api.get("/")
def handler(request, query: QueryParams = Query(...)):
    pass