Handle left work for cpp oauth2 client.

[jiazhai]

related with PR #7467 , some points need handle.

• [] It seems that in Java plugin we are using encoded auth param string but in c++ client, we don't use encoded auth param. Is there any reason?

• [] I think it is worth supporting reading client credentials from a JSON file. So applications can configure to read the credentials from a credentials file. We should support a similar feature in Java client similar as what we did for Token provider.

• [] We need to add documentation in c++ client.