Authentication : provide a way to use refresh_token mode in addition to password method

We had to connect to a Salesforce partner instance. Our partner did not provide login/password but instead a clientid clientsecret and refreshtoken.

Today in camel-salesforce component, authentication is forced to be login/password based.

This improvement should provide an alternative using refresh_token method of oauth.

Objective is to enrich the SalesforceSession.getLoginRequest(HttpConversation) method and add
fields.put("grant_type", "refresh_token");
as an alternative to
fields.put("grant_type", "password");

this has impacts on SalesforceLoginConfig class and other authentication involved classes.