This commit adds support for DNS Stateful Operations (DSO) as
specified in RFC 8490.
It adds `platform/dso_transport.hpp` header file which defines the
platform APIs/callbacks for DSO transport layer (e.g., DSN-over-TLS
or DNS-over-TCP).
The `Dso` module handles establishing connection with a peer, acting
either as a DSO client or server, establishing a DSO session over a
connection, and then sending and processing DSO request, response,
and unidirectional messages (including support for DSO TLV formats).
The `Dso` module also manages the session life cycle and timeouts,
namely the "Inactivity" and "Keep Alive" timeouts (including sending
and processing of Keep Alive messages when needed). It also handles
adding encryption padding before sending a message. It implements the
padding policy "Random-Block-Length Padding" from RFC 8467.
This commit also adds a detailed unit test `test_dso` covering the
behavior (including corner cases) of the `Dso` implementation. The
unit test provides an implementation of the DSO platform APIs which
emulate the DSO transport layer. It also includes a simplified alarm
platform implementation (emulating timers and allowing time to
advance in the unit test). These allow the unit test to cover more
complicated situations and behaviors (timeouts, failures, etc).
Abtin Keshavarzian