Add helper script for configuring auth extension

[scotttrinh]

For some frameworks, we might have enough information from environment variables to allow running a command to configure most of the auth extension for you, at least for local development. Depends on #824.

The idea would be to allow for things like:

npx @edgedb/auth-nextjs configure

To set things like the callback URL, allowed redirect URLs, and maybe other things that we can get a reasonable handle on by inspecting the environment.