Example Drogon configuration not matching the webviz schema

Custom keys in the example testdata configuration do not match the schema generated by `webviz schema`, and increases risk of users defining custom keys that we later want to use ourselves:
https://github.com/equinor/webviz-subsurface-testdata/blob/6a4b3e94f51ee93012fe43f290944f3d9e1b3474/webviz_examples/webviz-full-demo.yml#L18-L84

suggestion:
Include a new dict key like `custom_settings`, `app_settings` or etc where the users can define whatever they want, and which is included in the autogenerated `webviz schema` from `webviz-config` as a reserved key that we will not use for anything else later.

-> if included, we will need a PR in webviz-config

Start with webviz_examples/webviz-full-demo.yml lines 18-84 and compare its custom keys with the output of `webviz schema`. Then inspect the corresponding webviz-config schema behavior; done means the example uses an agreed reserved custom-settings key and the generated schema accepts it without reserving individual user keys.

Written by the indexing model from the issue text.