Introspection should be disabled separately from federated introspection

### What happened?

I wanted to disable introspection (`__schema` field only) on my server but it also disabled federated introspection (`service` field) because of https://github.com/99designs/gqlgen/blob/2c9f9c5f828bdda8aa03c4be7adf1d0c9abaa8f6/plugin/federation/federation.gotpl#L15-L17

### What did you expect?

Ideally we should be able to disable one without disabling the other.

### versions
- `go run github.com/99designs/gqlgen version`?

v0.17.35

- `go version`?

1.20.5

該 issue 指向 plugin/federation/federation.gotpl 的第 15-17 行，其中 introspection 和 federated introspection 目前是耦合的。首先，了解 gqlgen 外掛系統以及 introspection 的設定方式。查看 federation 外掛程式碼和主伺服器設定，以了解如何分離這些設定。檢查現有的 introspection 測試，以了解預期行為。目標是修改 federation 外掛，使其能夠停用 __schema，同時不影響 service 欄位。

由索引模型根據 Issue 內容生成。