remove `ActAs` role

I've noticed it is very hard to remove a role in this plugin, i.e. I can change the role to be `Yes or No answer` for example. but I cannot revert it to be the original chat gpt.

I think the most UX way to add this feature is to add a `regular` option in the `ActAs` window. but I've noticed the plugin fetches the roles from another github repository.

So I believe a machanism to add custom roles inside the plugin (maybe even let the user add custom roles) would be a great feature.

Start with the ActAs window and the code that fetches roles from the external GitHub repository. Determine how a regular role and plugin-defined or user-defined roles should be represented and persisted. Done should allow users to revert to the normal ChatGPT behavior and select custom roles without relying only on the remote role list.

Written by the indexing model from the issue text.