Use billing subdomain to simplify dashboard for custom domains

[jakubno]

Description

Billing service runs on billing. subdomain, so you can pass only 1 url now

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Latest commit: b72817d

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[mishushakov]

I think domain sounds confusing, maybe use apiHost? or something that should tell it is hostname/url/address for the API

[jakubno]

It isn't api url, it's the base url:
domain: e2b.dev
api url: https://api.e2b.dev