Load roles from OAuth2

I'm managing all users with SSO/OIDC/OAuth2 provider and there I also have groups/roles etc.

It looks like currently using `connector-basic` plugin it's not possible to load roles like Admin/Moderator from OAuth2 provider. This means after each user logins I would have to assign them that manually.

In my provider it has them like this:
```
"groups": [
"Administrators"
]
```

该 payload 只提到了 connector-basic 插件和一个 OAuth2/OIDC provider group claim，没有文件或测试。首先定位插件的登录和 claims-mapping 入口，然后明确 `groups` 值应如何映射到 Apache Answer 角色；当 Administrators 等角色能够在登录时无需手动分配即可加载时，就算完成。

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