Add a property to the resolver config to allow for a full resolvergen configuration

### What happened?

I need to be able to run gqlgen and pass in a template for generating a resolver.

I need to be able to pass in new imports and make significant code additions (maybe even some convenience methods). This will allow my implementing team to follow my guidlines.

### What did you expect?

I thought the config would have an option to just replace the whole gotpl file... it doesn't.

### Minimal graphql.schema and models to reproduce

```
type Query {
resolver: Resolver!
}

type Resolver {
name: String!
}
```

```
schema:
- "testdata/schema.graphql"

exec:
filename: testdata/singlefile/out/ignored.go
model:
filename: testdata/singlefile/out/generated.go
resolver:
type: CustomResolverType
layout: follow-schema
dir: testdata/resolvertemplate/out
filename_template: "{name}.resolvers.go"
resolver_template: "testdata/resolvertemplate/customResolverTemplate.gotpl"

models:
Resolver:
model: github.com/99designs/gqlgen/plugin/resolvergen/testdata/singlefile/out.Resolver
```

查看代码库中的 resolvergen 插件配置，可能位于 plugin/resolvergen/config.go。了解当前模板系统的工作方式，以及需要在哪里添加 resolver_template 字段。检查 resolvergen 的现有测试，了解配置是如何验证和使用的。此更改涉及扩展配置结构体并更新模板加载逻辑。

由索引模型根据 Issue 内容生成。