Discuss: Change format of `AUTH_API_KEY` so `URL` is first

While working on the `get_baseurl_from_auth_api_key/0` function in https://github.com/dwyl/auth_plug/issues/214
I was reminded of the format of the `AUTH_API_KEY` is:

```sh
"2cfxNb3PwJuibpFGfZLFRxw3B18oykn4PiJMinv4L1D/2cfxNarsZzzwNzy9B49417bueRp9D5st/authdemo.fly.dev"
```

And it got me thinking _why_ did we/I put the URL after the public/private keys?
Wouldn't it make more sense to have it as:

```sh
"authdemo.fly.dev/2cfxNb3PwJuibpFGfZLFRxw3B18oykn4PiJMinv4L1D/2cfxNarsZzzwNzy9B49417bueRp9D5st"
```

And we could then make this URL _valid_ and render the `/app`.
I don't see a reason _not_ to, other than introducing a **breaking change**.
But given that the rebuild will be total and require new keys for all apps we should consider it. 💭

Start with the get_baseurl_from_auth_api_key/0 function referenced in the linked auth_plug issue, then trace how AUTH_API_KEY values are generated and consumed. Compare the existing and proposed URL-first formats, including the breaking-change and new-key implications; done means the format decision and migration requirements are documented.

Written by the indexing model from the issue text.