JSON Schema for Panel composables only have options and fieldConfig

[Duologic]

There is work being done in Thema to solve the composition issue.

Awaiting work in Thema to land, I'd like to move the logic that now sits in Grafonnet up the chain into Grok so we have complete JSON schema's for Panels as the interest in the JSON schema's has been increasing.

Logic in Grafonnet can be found here: https://github.com/grafana/grafonnet/blob/c7910c7366fe308ea6bc00fc365f953ac5f98990/grafonnet-base/main.libsonnet#L174