[Feature request] Custom relays

In addition to following tags and instance relays, it would be nice to be able to create a customizable relay which includes various tags and/or instances.
With options such as:
- filters (language, keyword, instances)
- private (can only be followed by select users) or public (anyone can follow)

Similar to Bluesky custom feeds.

This would help tremendously with content discoverability, especially for new users as you could share a curated relay with them .
It would require an account management page to manage the custom relays.

Start by reviewing how tag and instance relays currently work, then identify the account management entry point needed to create and manage custom relays. Define the scope for filters, public versus private following, and the account management page before implementation; done means users can create, manage, and follow the specified custom relays.

Written by the indexing model from the issue text.