Change SSE response structure so that it's compatible with GraphQL SSE standard

[perzanko]

Description
The current payload of a subscription response in the SSE protocol looks like this.

{"data": {."foo": "bar" } }\n\n

It is simply encoded JSON with a new line.

The proposed response format is as follows:

event: next\ndata: {"data": {."foo": "bar" } }\n\n

The encoded response additionally contains an event segment event: next and the data is prefixed with a data: field, indicating data segment. This structure is consistent with GraphQL over Server-Sent Events Protocol specification and official SSE standard.

Reason
Many graphql clients expect a response consistent with the above standard. An example of this is graphql-mesh, which expects responses from subgraph services sent in a structured manner. Or even the highly popular client graphql-sse used in browsers also requires this format.

⚠️ Please note
This is a breaking change and should be implemented with backward compatibility. Perhaps introducing an additional configuration option or an option to override the response format would be appropriate in this case, perhaps even more appropriate.

[perzanko]

@benwilson512 😉☝️

[benwilson512]

Hey @perzanko thanks for the PR!

As you note, this is a backwards incompatible change. I think we should support some sort of flag on probably the Absinthe.Plug plug that we can use to switch between the types of return shapes.