[Pairing Adapter] - Allow to specify OAuth Style

[aszecowka]

In the current implementation, ClientID and ClientSecret are sent as a basic auth in the Authorization header.
Client ID and secret must be urlencoded in the authorization header, according to the RFC 6749 and our library is doing so: golang/oauth2@13449ad

Unfortunately, there are OAuth servers that do not decode those values (https://github.com/cloudfoundry/uaa), and as a result, we will get 401 Unauthorized Response: golang/oauth2#320.

As a workaround, client_id and client_secret can be sent in the body, and then this problem does not occur.

cc := clientcredentials.Config {	
		AuthStyle: oauth2.AuthStyleInParams,
}

Possible solution: Make auth style configurable in the Pairing Adapter via environment variable.