Client libraries to generate fallback hosts from custom environment

[QuintinWillison]

This enhancement will automatically use fallback hosts, deterministically derived from the supplied environment, when they've not been specified explicitly.

It will solve the problem whereby customers ship their app built against our client library, seeing that it works with basic testing, but don't realise that it will quickly stop working when things go wrong with the primary host.

See the internal issue which proposes this enhancement.

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