Add support for custom validators in default GraphQL backend

The default GraphQL backend seems to perform validation prior to execution of a query. The validators that are being run are statically defined in https://github.com/graphql-python/graphql-core/blob/fe703f0d176c78b205d2abf929193f1e910fe4a0/graphql/validation/rules/__init__.py#L32-L57 and there doesn't seem to be a very good way to add custom validators to this list.

It would be nice to be able to extend the default GraphQL backend with your own validators, e.g. by having a overridable function called `get_validators` in the backend class that's used for getting the validators.

Right now the only option (as far as I'm aware) seems to be to completely re-implement the execution and validation logic if you don't want to monkeypatch the validators list.

まずデフォルトの GraphQL バックエンドの検証および実行パスを特定し、次に issue で参照されている graphql/validation/rules/__init__.py のバリデーターリストと比較します。カスタムバリデーターが検証と実行を再実装せずにデフォルトのバックエンドを拡張できるようになれば、作業は完了です。

索引モデルが issue の本文から書いたものです。