New lead field and beta functionality

As of October 7 2020, we're introducing BETA functionality that allows modifying the leads through API v2. Currently it's available only in sandbox environment.

As a result, the model for Lead changes new field (enum) is introduced - LeadType{BLOCK,INQUIRY,BOOKING}.

The Lead model is being changed also in V2 in production environment.

Feel free to review leads documentation for v2 under https://dev.hostfully.com/v2.0/reference#leads-get for more details.