OAuth TokenHandler should check Authorization header for client credentials

Description

Currently, TokenHandler assumes that the Token request's body contains client credentials. However, some OAuth requests would contain client credentials in Authorization header:

In this case, it would throw ValidationError even though client credentials are provided in request header.

Can we add a fallback such that if client_id is not found in formData, we try to get it from header? e.g.

async def handle(self, request: Request):
    try:
        form_data = dict(await request.form())

        # Try to get client credentials from header if missing in body
        if "client_id" not in form_data:
            auth_header = request.headers.get("Authorization")
            if auth_header and auth_header.startswith("Basic "):
                encoded = auth_header.split(" ")[1]
                decoded = base64.b64decode(encoded).decode("utf-8")
                client_id, _, client_secret = decoded.partition(":")
                client_secret = urllib.parse.unquote(client_secret)
                form_data.setdefault("client_id", client_id)
                form_data.setdefault("client_secret", client_secret)

        token_request = TokenRequest.model_validate(form_data).root
    except ValidationError as validation_error:
        return self.response(
            TokenErrorResponse(
                error="invalid_request",
                error_description=stringify_pydantic_error(validation_error),
            )
        )
    ...

Thanks.

References

No response