Rotate hosts

[rafaeelaudibert]

Rather than accepting a single host, some users are interested in accepting more than one host to try and circumvent ad blockers.

The idea is that you'd be able to say posthog.init(<token>, { api_host: ['proxy.posthog.com', 'proxy.third-party.com'] }) and the library would be able to detect that the first request failed, and then attempt a second one against the fallback host.

This is helpful for people that are using our self-provisioned proxy but that are still seeing some usage being blocked, which would then fallback to their self-hosted proxy in something like Vercel, where they're charged by the request. If they can offload most of the requests to us, then their Vercel bill will be much lower.