Allow reloading hooks to support whole-extension reloading

[chrstnb]

We now have support for hooks in extensions (#14460), however will need to support reloading them as with MCP servers in #12239 and context + custom commands (as linked here #11108)

[gemini-cli]

Found possible duplicate issues:

• Extension reloading in the CLI #11108
• Refresh custom /commands and extensions without restarting Gemini CLI #8317

If you believe this is not a duplicate, please remove the status/possible-duplicate label.