[Help] Example of Apple SignIn implementation with POST_FORM
Hi 😄 ,

I've would like to implement Apple SignIn authentication with Grant + Purest, but I'm unable to find great configuration with Purest (not update since 4 years with latest providers like 'Apple')

`Grant` is unable to read POST_FORM `access_token` and it's return `oAuth2 missing code parameter` error.

Grant config (not sure about this)
```js
 apple: {
      key: '', // reverse.domain.name
      secret: '', // Need to be refer with "Generate client secret" 
      callback: `[domain.name]/auth/apple/callback`,
      nonce: true,
      state: true,
      scope: ['name', 'email'],
      custom_params: {
        response_type: 'code id_token',
        response_mode: 'form_post' // necessary... Apple constrain 😢 
      },
    },
```

Purest configuration testing :
```js
const apple = purest({
        provider: 'apple',
        config: {
          apple: {
            'https://appleid.apple.com': {
              __domain: {
                auth: {
                  auth: { bearer: '[0]' },
                },
              },
              '{endpoint}': {
                __path: {
                  alias: '__default',
                },
              },
            },
          },
        }
      });

apple
        .query()
        .post('auth/token') // not sure about this ? apple want post request only
        .auth(access_token)
        .request((err, res, body) => {  
              if (err) {
                   callback(err);
              } else {
                  // let do some stuff...
                  callback(null, {body});
              }
        });

```

I've successfully sign in with other libraries (`apple-signin` and `apple-auth`) but I would like to perform signIn request only with Grant + Purest and not implement a tierce package.
But `Grant` return the same error with the `access_token` (`code` for Apple) and it's unable to return and read it from POST_FORM request. 

The project "Playground" implement Apple SignIn testing example, but I can't find any code to understand how it's working.
Apple documentation is clearly... pitiable.

Thanks for your help !