Add support for custom loader params

**Is your feature request related to a problem? Please describe.**

My primary goal is to hide certain enum values from the generated types. For the schema like:

```graphql
enum MyEnum {
FIRST
SECOND
THIRD @ignore
}
```

I would like to generate Typescript enum with just `FIRST` and `SECOND`. I figured out that the best way to achieve that is a [custom loader](https://www.graphql-code-generator.com/docs/getting-started/schema-field#custom-schema-loader).

I need a way to tell the custom loader that my app is ready to support `THIRD` value and `@ignore` directive should no longer be respected for that particular value.

**Describe the solution you'd like**
```js
generates: {
schema: {
'my-schema.graphql': {
loader: './schemaLoader.js',
options: {
supportedEnums: ['MyEnum.THIRD']
}
}
}
}
```

and then in the loader
```js
module.exports = function (schemaString, config) {
console.log({ supportedEnums: config.loaderOptions.supportedEnums })
```

**Describe alternatives you've considered**

#### Sharing global config with the loader

Expose a global config to the custom loader
```js
generates: {
schema: {
'my-schema.graphql': {
loader: './schemaLoader.js'
}
},
config: {
supportedEnums: ['MyEnum.THIRD']
}
}
```

and then in the loader
```js
module.exports = function (schemaString, config) {
console.log({ supportedEnums: config.SOME_NAMESPACE.supportedEnums })
```

#### Another config file
A custom schema loader can load it's config from the file system like

```js
module.exports = function (schemaString, config) {
const configFilePath = `${schemaString}.json`
// Read supportedEnums from the config file
```

This way no changes are required to the generator, although it creates one more config file.