Send EventGrid spec

[lmazuel]

After a lot of investigation and discussion with all party involved, drafting a spec for send of EventGrid. We might be near to be done with @t-swpill work, but for clarity needs to be written:

We shall have ONE send method only, that accepts different types of content:

• A CloudEvent instance
• A EventGridSchema instance
• A dict, or any type
• An explicit list of the three preceding format

For the later, we shall send the dict "as-is" on the wire, and not try any kind of client-side validation. Example, this is a valid way to send a Cloud Event:

client.send({
   "id": "foo",
   "source": "blo",
   "specversion":"1.0",
   "type": "blie",
   "data": {"joy":True}
})

The dict follow the CloudEvent spec, so there is no reason to not send it that way.

Implementation (important) detail: when sending a CloudEvent, we must set the Content-Type to "application/cloudevents-batch+json; charset=utf-8". This means in the dict case, we must detect if the dict is Cloud Event. We recommend using a predicate that checks:

• All required Cloud Event attributest are provided: id/source/specversion/type
• specversion has the value "1.0"

If the dict follow this structure, we send the JSON with application/cloudevents-batch+json; charset=utf-8, otherwise we use application/json

In case we want to be explicit:

client.send({
       "id": "foo",
       "source": "blo",
       "specversion":"1.0",
       "type": "blie",
       "data": {"joy":True}
   },
   content_type="application/json"
)

And list are supported:

client.send([{
   "id": "foo",
   "source": "blo",
   "specversion":"1.0",
   "type": "blie",
   "data": {"joy":True}
},{
   "id": "foo",
   "source": "blo",
   "specversion":"1.0",
   "type": "blie",
   "data": {"joy":False}
}])

CC @KieranBrantnerMagee @rakshith91