fix(connector): move plugin initialization into PluginLoader for lazy init

[pingsutw]

Summary

• Move clientSet, connectorRegistry, plugin struct creation, and watchConnectors call from newConnectorPlugin into the PluginLoader callback
• This enables lazy initialization — connector clients are only created when the plugin is actually loaded, not at registration time
• Use cfg.SupportedTaskTypes directly for the initial SupportedTaskTypes field instead of fetching from the registry at startup

Test plan

• Existing unit tests pass for the connector plugin
• Verify connector plugin loads correctly and registers supported task types at runtime

• main
  • Flyte 2 WIP #6583
    • fix(connector): move plugin initialization into PluginLoader for lazy init #6932 👈