Expose client's RemoteIP in TIdHTTPServer.OnCreateSession event

[rlebeau]

Expose access to the client's RemoteIP in the TIdHTTPServer.OnCreateSession event. It is already cached in TIdHTTPRequestInfo and used when creating default HTTP sessions, but is not exposed when users want to create their own sessions, without resorting to accessing the socket's TIdSocketHandle.PeerIP directly. To expose the RemoteIP without changing the event signature or defining a new event, a solution could be to define a custom Context class instead (see #227).