[Enhancements] Allow users to choose a specific frontend

[TheFrenchGhosty]

Is your feature request related to a problem? Please describe.

The instances will redirect to all frontends available for a service (so Invidious and Piped for YouTube), the user can't choose a specific frontend and is forced to either use both (if they want a random instance everytime) or specify an instance URL (which mean that it will always be this instance)

Describe the solution you'd like
The user should be able to choose a frontend,

Libredirect implementation for reference (which is one of the good thing about Libredirect):