[FEATURE]: Allow plugins to have their own configuration

[airtonix]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently we can use plugins like:

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": ["opencode-skills", "opencode-sessions"],
}

But it occurs to me that some plugins would benefit from allowing the user to configure them differently:

{
  "$schema": "https://opencode.ai/config.json",
  "plugin": [
    { "name": "opencode-skills", "config": { "paths": [ "~/Dotfiles/OpencodeSkills" ] } },
    "opencode-sessions"
  ],
}