Add file/package configuration options when generating service.go

This patch adds the option for users to specific the filename and/or
package of the `service.go` file. To configure, users will need to add a
new section like the following to their `gqlgen.yml` file similar to the
ones that exist now for `exec` and `resolver:

```
service:
 filename: generated/service.go
  package: generated
```

codegen/config/config.go

@@ -23,6 +23,7 @@ type Config struct {
 	Exec              PackageConfig              `yaml:"exec"`
 	Model             PackageConfig              `yaml:"model"`
 	Resolver          PackageConfig              `yaml:"resolver,omitempty"`
+	Service           PackageConfig              `yaml:"service,omitempty"`
 	AutoBind          []string                   `yaml:"autobind"`
 	Models            TypeMap                    `yaml:"models,omitempty"`
 	StructTag         string                     `yaml:"struct_tag,omitempty"`
@@ -39,6 +40,7 @@ func DefaultConfig() *Config {
 		SchemaFilename: StringList{"schema.graphql"},
 		Model:          PackageConfig{Filename: "models_gen.go"},
 		Exec:           PackageConfig{Filename: "generated.go"},
+		Service:        PackageConfig{Filename: "service.go"},
 		Directives:     map[string]DirectiveConfig{},
 	}
 }
@@ -236,13 +238,19 @@ func (c *Config) Check() error {
 			return errors.Wrap(err, "config.resolver")
 		}
 	}
+	if c.Service.IsDefined() {
+		if err := c.Service.Check(); err != nil {
+			return errors.Wrap(err, "config.service")
+		}
+	}
 
 	// check packages names against conflict, if present in the same dir
 	// and check filenames for uniqueness
 	packageConfigList := []PackageConfig{
 		c.Model,
 		c.Exec,
 		c.Resolver,
+		c.Service,
 	}
 	filesMap := make(map[string]bool)
 	pkgConfigsByDir := make(map[string]PackageConfig)
@@ -378,6 +386,12 @@ func (c *Config) normalize() error {
 		}
 	}
 
+	if c.Service.IsDefined() {
+		if err := c.Service.normalize(); err != nil {
+			return errors.Wrap(err, "service")
+		}
+	}
+
 	if c.Models == nil {
 		c.Models = TypeMap{}
 	}

plugin/federation/federation.go

@@ -209,8 +209,8 @@ func (f *federation) GenerateCode(data *codegen.Data) error {
 	}
 	return templates.Render(templates.Options{
 		Template:        tmpl,
-		PackageName:     data.Config.Exec.Package,
-		Filename:        "service.go",
+		PackageName:     data.Config.Service.Package,
+		Filename:        data.Config.Service.Filename,
 		Data:            f,
 		GeneratedHeader: true,
 	})