* lib(messages): Implement a 4.0 Messages and other supporting interfaces
This commit introduces the Message, Serializer and Deserializer, and Binding
interfaces used to convert a CloudEvent into a Message that can be sent across
a transport protocol. The first protocol implemented for this is HTTP, and some
of the functionality formerly in src/transport/http has been simplified,
reduced and/or moved to /src/messages/http.
Test for V1 and V3 events are in place. Conformance tests have been modified to use
these new interfaces vs. the HTTP Receiver class.
Signed-off-by: Lance Ball <lball@redhat.com>
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