[Improvement]: Make custom domains a non-pro feature

[joshka]

Describe the improvement

As a free user of heroku and then fly.io, I was able to point a CNAME at a host and spin up a small low resource projects that have a name that represents the project rather than the hosting company. It would be nice to be able to do this with Shuttle so I can move away from using Ruby for these sorts of projects.

E.g. https://xkcd-with-alt-text.joshka.net/ is a small proxy in front of xkcd's rss feed that moves the alt-text to the description rather than forcing the user to hover over images to see it.

To be clear, any sort of amount (especially not $120/yr) for these sorts of apps will never be worth it.

Custom domains seems like a feature that shouldn't (once setup correctly) add any measurable incremental load on shuttle's resources or support burden on its developers, so it's difficult to see how this makes sense as a charged feature rather than a free one.

There is similar sentiment expressed in the discord for this functionality.

josh: Thanks Joshka, it definitely looks like there is overwhelming demand for custom domain support 😅
-- https://discord.com/channels/803236282088161321/953005742842069013/1185919864624381973

Duplicate declaration

• I have searched the issues and this improvement has not been requested before.