Add hints to generics in Route.respond

[JacobHayes]

The json parameter to Route.respond is missing hints for the List and Dict elements, which makes pyright sad in strict mode:

.../tests/test_url.py:6:5 - error: Type of "respond" is partially unknown
    Type of "respond" is "(status_code: int = 200, *, headers: Headers | Dict[str, str] | Dict[bytes, bytes] | Sequence[Tuple[str, str]] | Sequence[Tuple[bytes, bytes]] | None = None, cookies: Dict[str, str] | Sequence[Tuple[str, str]] | Sequence[SetCookie] | None = None, content: str | bytes | Iterable[bytes] | AsyncIterable[bytes] | None = None, text: str | None = None, html: str | None = None, json: str | List[Unknown] | Dict[Unknown, Unknown] | None = None, stream: SyncByteStream | AsyncByteStream | None = None, content_type: str | None = None, http_version: str | None = None, **kwargs: Any) -> Route" (reportUnknownMemberType)
1 error, 0 warnings, 0 informations

This PR just adds the hints for the elements. I think python's json.dump will accept arbitrary key types, but the JSON spec only defines strings so I used Dict[str, ...]. I'm happy to change that to Any if you'd prefer.

Thanks for the helpful library!