Support equivalent of VSCode User Settings (JSON)

[dominiccooney]

VSCode represents user settings in UI, but also as a JSON blob. The extension controls a lot of experimental settings through that blob. When doing development, being able to set that blob would mean you need to run the extension in the debugger less.

For example, this would have enabled testing codegemma:2b completions and ollama chat in JetBrains without having to hand out custom builds of the plugin.

We manually enumerate some settings in JetBrains UI, and support a limited set of properties by writing keys in an obscure file. Clean those up, or at least, don't break them.

• Add settings UX somewhere obscure where people can blat JSON into a field, route it to getConfiguration.
• Generalize the getConfiguartion hook @steveyegge added so it is "JSON" (at least Map<String, Any>) not string values.
• Fix the agent so that it understands config is navigating a JSON structure, not a set of flat keys with dotted names. (The way agent does it is not how VSCode works.)
• Unify the existing settings overrides with the updated agent mechanism.