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
```