Proposal for user system and user configuration

### What feature is it?

Currently, each RSSHub instance has a fixed global configuration. Users who wish to customize their configurations can only do so by self-hosting their own RSSHub instance. However, many users lack the technical ability to self-host, which hampers the advancement of RSSHub.

For example, numerous users have recently inquired about subscribing to Twitter feeds. The only solution at present is self-hosting.

Therefore, I propose implementing a user system that enables users to log in and set up personal configurations to override the global configurations. This way, they can use their private addresses for subscriptions.

### What problem does this feature solve?

As mentioned above.

### Additional description

This feature offers the benefit of providing our official instance with a profit model to cover high operating costs and project development costs. However, it may also have the drawback of encouraging users to become overly dependent on the same instance.

### This is not a duplicated feature request or new RSS proposal

- [X] I have searched [existing issues](https://github.com/DIYgod/RSSHub/issues) to ensure this feature has not already been requested and this is not a [new RSS proposal](https://github.com/DIYgod/RSSHub/issues/new?assignees=&labels=RSS+proposal&template=rss_request_en.yml).