laziness

[proppy]

The library should give a way to construct Firebase call independently from executing them.

That could enable:

• keeping static const version of repeated request.
• fine grained control over HttpClient instance used to execute request.
• overload for testing, portability and debugging

That could be done by adding FirebaseCall::execute(HttpClient&) and having the default constructor call it directly to preserve the easy Firebase.push/get/* flow, while still allowing user to call it manually when constructing FirebasPush/FirebaseGet/* object manually with two step initialization (see #58)

[proppy]

This should be provider by the underlying API (not the arduino wrapper), see #70