Refactor plugin config and cache path (#13333)

Update config.toml plugin entries to use
<plugin_name>@<marketplace_name> as the key.
Plugin now stays in
[plugins/cache/marketplace-name/plugin-name/$version/]
Clean up the plugin code structure.
Add plugin install functionality (not used yet).

codex-rs/core/src/config/types.rs

@@ -778,7 +778,6 @@ pub struct SkillConfig {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema)]
 #[schemars(deny_unknown_fields)]
 pub struct PluginConfig {
-    pub path: AbsolutePathBuf,
     #[serde(default = "default_enabled")]
     pub enabled: bool,
 }