Support deserialization of arbitrary payloads

[gkinsman]

The goal of this issue is to be able to handle non-JustSaying event payloads.

Possible implementations

1. Add a implementation of IMessageSerializer that supports cloud events and deserializes into a JustSaying.Models.Message. Downsides include consumers consuming a JustSaying Message even though the input is a cloud event.

2. Revisit removing Message base type and removing the link between Subject and Type to determine which serializer to use.

3. Add new API to specify a subject and serializer tuple. When a message arrives, find the serializer by subject, and use that to produce a MessageWithAttributes. Serializers would be created per registration.

• Bigger change than 1, not as big as 2

[NavaneeswarManam]

#944 (comment)