Accept single bare values for list-typed query parameters

[semohr]

validate_querystring only produces a list when a key occurs more than once. For a field declared as list[int], a client sending a one-item array gets a 400 instead of [1].

This PR enhances validate_querystring to treat a single occurrence of a parameter declared as a list as a one-element list, based on the model's annotations. It works for all model kinds and keeps the existing [] suffix and repeated forms working.

The following requests are now treated equally:

/?elems=2             -> 200 {"elems": [2]} # This was not possible without workaround before
/?elems[]=2           -> 200 {"elems": [2]}
/?elems=2&elems=3     -> 200 {"elems": [2, 3]}
/?elems[]=2&elems[]=3 -> 200 {"elems": [2, 3]}

---

Details

Before, accepting a single value for a list field required special pydantic syntax (a ``BeforeValidator`` per field), which is now unnecessary (also wasnt really possible with msgspec):

from typing import Annotated

from pydantic import BaseModel
from pydantic.functional_validators import BeforeValidator
from quart_schema import validate_querystring

def _to_list(value: str | list[str]) -> list[str]:
    if isinstance(value, list):
        return value
    else:
        return [value]

class Query(BaseModel):
    keys: Annotated[Optional[List[str]], BeforeValidator(_to_list)] = None

@app.route("/")
@validate_querystring(Query)
async def index(query_args: Query):
    ...

[semohr]

@pgjones Short ping for visibility.

Was thinking this is a good addition as it simplifies this quite common usecase imo.