Server: support creating new ExchangeContext for other protocols

[holbrookt]

Problem

Server.cpp hides access to ExchangeManager and does not provide any public methods for creating a new ExchangeContext or adding other message handlers.

This means a device using Server.cpp cannot start, for example, a BDX exchange (which blocks development of OTA Requestor).

Proposed Solution

One of the following:
A) Inject dependencies by adding ExchangeManager and others to Server init arguments (instead of being static inside Server.cpp)
B) Provide an access method for getting a pointer/reference to the ExchangeManager
C) Provide a public method that returns a new ExchangeContext for a given protocol