[Feature request] Include route hints for private channels

I would like to propose an important feature for that use private channels.
For which situation?
- when the Alby Hub user is having multiple private channels
- when user want to receive a payment to a specific private channel, so will need to add routing hints to his LN invoice, selecting one of his LSP channels.
- this option to be available for all LN implementations used as funding source for an Alby Hub

The issue does not name files, tests, or an entry point. Start by tracing how Alby Hub creates LN invoices and represents funding sources, then determine how a user-selected LSP channel could provide routing hints across each supported LN implementation. Done means private-channel payments can target the selected channel consistently.

Written by the indexing model from the issue text.