CloudEvent must be an abstraction of the spec, and not JSON specific

[lmazuel]

Again, I'm not saying the current prototype is incorrect, but writting this issue so I'm sure we address the spec explicitly.

Cloud Event is an abstraction of what is an event: https://github.com/cloudevents/spec/blob/v1.0/spec.md

As so, our CloudEvent class should not assume any serialization of a CloudEvent concept, and model the concept without thinking about the serialization. This implies:

• There should be no data_base64 attribute, but helpers to set bytes correctly
• Extensions should be treated as explicit extensions (and not simple new attributes at the base level).

Example:

c = CloudEvent(
    id=12,
    source='MyApp',
    type='CreateStuff'
)  # specversion is set by default to "1.0"

c.subject = "mynewfile.jpg"  # I could have set it as kwargs, but for the sake of sample

# Set data. API here could be discuss, we could use `data` property setter for instance
c.set_bytes_data(b"PDF%...", "application/pdf")

# Extensions are allowed aside, to avoid collision
c.extensions["reasonCode"] = 204

c.model # Returns the deserialized model

A JSON serialization of this would be:

{
    "id": 12,
    "source": "MyApp",
    "type": "CreateStuff",
    "subject": "mynewfile.jpg",
    "datacontenttype": "application/pdf",
    "data_base64": "ABCDEF_BASE64_OF_PDF",
    "reasonCode": 204
}

While a binary mode serialization would be HTTP request

ce-id: 12
ce-source
ce-source: MyApp
ce-type: CreateStuff
ce-subject: mynewfile.jpg
ce-datacontenttype: application/pdf
ce-reasonCode: 204

PDF%...

[KieranBrantnerMagee]

Notes from our OOB discussion:

Extensions:

• take as kwargs, potential risk overlap with data_base64/data args.

Binary vs. json serialization:

• kwarg overload in send() to let control of this. We assume b64 in absence of any guidance since it "will just work" consistently.
  • Have enum for overload type, e.g. JSON or binary serialization mode.
• won't need set_bytes_data since this will be handled at send time.
• data as string? send plain. bytes? b64 encode and send. (with optional overload.)
  • this is gonna be wacky in 2.7. Perhaps "if unicode send directly; if otherwise encode and bytes."

c.model: StorageBlobDeletedEvent, keep in mind this may shift as we adjust the "GetData" discussions.