[Feature Request]: 为 GraphQL Subject.Character 添加 actors

https://api.bgm.tv/v0/graphql
```graphql
fragment SubjectFragment on Subject {
id
characters {
type
character {
id
actors # expected
}
}
}

query BatchGetSubjectQuery {
s0:subject(id: 425991){...SubjectFragment}
}
```

HTTP API 支持获取 actors, 希望为 GraphQL 也增加

Start from the GraphQL endpoint and the SubjectFragment/BatchGetSubjectQuery example in the issue, then trace the Subject.Character schema definition. Add actors so the shown query can retrieve them, and verify that the query returns actors as the HTTP API already does.

Written by the indexing model from the issue text.