Add BetterAuth support to the Convex Cli

[primerch]

The current Convex + TanStack Start cli template has auth options for Clerk and WorkOS, but nothing for BetterAuth.
Now that Convex officially supports BetterAuth (through @convex-dev/better-auth), it would be great to have a built-in option for it in the template.

Right now, anyone wanting BetterAuth has to follow the separate guide here:
[https://convex-better-auth.netlify.app/framework-guides/tanstack-start]
And manually set everything up. That means juggling multiple sources when starting a new project, which gets messy fast.

I've tried a few approaches myself:

• The default Convex CLI setup
• Straight from the BetterAuth docs
• The TanStack Start cli — this is the one I went with, it has more options to choose from which reduce my initial setup time, eg, tailwind, tooling, hosting eg..

It works, but having it included out of the box would save a lot of time and confusion.
Thanks!