[fix] rename BASE to ORIGIN and fix config handling

[Conduitry]

This should fix #3420 and fix #3424, but also makes some other changes.

The defaults listed in the readme were out of date, and they have been updated.

This also renamed BASE to ORIGIN, per a conversation I had with Rich on Discord.

It also adds 'ORIGIN' as a default value for env.origin (previously it was undefined), to align with the readme, which indicated that you should be able to just do that without any build-time configuration.

I'd like for there to be fewer of these build-time defaults in the adapter config, but I understand that this might not be very popular.

Although the changes are pretty straightforward, this hasn't been properly tested yet. Because of this, and the decision I'd like to come to about the default env/header behavior, I haven't added changesets yet and I am marking this as draft.

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[benmccann]

I'd like for there to be fewer of these build-time defaults in the adapter config, but I understand that this might not be very popular

I'm not sure what you're suggesting, but I'm not sure it'd be unpopular. I'm happy to hardcode the environment variable names. I don't know if you were suggesting something else though?

[benmccann]

the rename to origin is a great change!

[Conduitry]

I was suggesting there being no default for these environment variables, but that they're still configurable, which would mean you would need to explicitly configure them at build time so that you could specify these values at runtime.

[benmccann]

Ok, then I agree that'd be very unpopular 😆 At least with me