Optional Message Model

[dbluhm]

We should provide a Message class that can be used by users of this library. However, we should not require that the included Message is the only way to interact with the library.

This essentially means that we should operate on messages as dictionaries.

Validation. I think we should keep validation to a practical minimum at the interface layer. According to the DIDComm v2 spec, only the following attributes are required:

• id
• type
• body

The Message class MAY provide additional validation; the interface itself, however, MUST NOT validate any deeper than this.