Add media types for CMW

[thomas-fossati]

The document should define an application/cmw media type to be used when CMWs are carried by transports that use media types to identify their payloads. E.g.:

POST /verify HTTP/1.1
Host: verifier.example
Content-Type: application/cmw+json

[
  "application/eat+jwt",
  "eyJhbGciOiJFUzI1NiIsInR5cCI6IkpXVCJ9..."
]

Note that #39 should provide a similar application/cmw-collection media type to be used for composite evidence formats that use a CMW collection as their encapsulation:

POST /verify HTTP/1.1
Host: verifier.example
Content-Type: application/cmw-collection+json

{
  "cpu": [
    "application/eat+jwt",
    "ZXlKaGJHY2lPaUpGVXpJMU5pSXNJblI1Y0NJNklrcFhWQ0o5Lg..."
  ],
  "gpu": [
    "application/ucs+json",
    "eyJlYXRfbm9uY2UiOiAiamtkOEtMLTg9UWx6ZzQifQ"
  ]
}