Fix EXPLAIN

[benjie]

1. Currently EXPLAIN ANALYZE can only be used on SELECT statements. This isn't quite right since a volatile function can be called from a SELECT statement - explain analyze should only be available on non-mutation SQL queries.
2. Currently you have to set an environmental variable to see explain in GraphiQL. This should instead be controlled by a header (and a config option). And there should be separate configs for EXPLAIN and EXPLAIN ANALYZE
3. It should be easy to copy your query plan into a cost analysis tool such as https://explain.depesz.com/ or https://explain.dalibo.com/
4. It should be easy to copy the SQL and potentially even embed the variable values so the user can debug.
5. Rather than sending mermaid diagrams over the wire, the system should generate a JSON representation of the plan and the mermaid diagram (or alternative!) should be generated on the client

[jemgillam]

If we don't do this before release, we need to make it clear that the data format is not stable.

[jemgillam]

cf #413