Simplify `sse` support in AsyncAPI specs

[jfallows]

Describe the desired outcome from the user's perspective
As an AsyncAPI author, I want to be able to use Zilla to map sse to kafka using AsyncAPI v3 syntax.

Acceptance criteria

• Syntax fits naturally with http AsyncAPI specs

Additional context
None

Tasks

• simplify to use x-zilla-sse operation binding instead of sse protocol
  • update asyncapi binding accordingly
  • update asyncapi specs in zilla-demos etc
• propose final x-zilla-sse syntax as sse support in AsyncAPI v3 spec