- Subscriptions can be executed only with SSE.
- Queries and mutations can be executed with HTTP or CLI.

This makes it possible to the execute a prepared subscription operation
upon receiving each event.
Also: make it possible to return 'aux' data.

It uses lasse, just like the REST SSE handler

- Handle SSE requests under the '/sse' subpath
- Export utils used by the SSE handler
- Allow passing parameters with GET (qs) or POST (body).
  This follows the GraphQL specs, and allows to handle SSE.

- unsupported operation
- invalid query string

This common code is used by REST and GraphQL SSE handlers.

This is actually used only for building the queries for tests.
The commands will be displayed in the CLI help - we can change this
later if needed.

Put the more generic tests (mostly error handling) in a separate suite.