Formalize host services SDK

[autodidaddict]

Proposed change

Host services is the set of functionality provided to Nex workloads as a built-in convenience. For example, JavaScript functions get access to a key-value bucket, an object store, basic pub/sub, and an HTTP client.

This would extract host services client code into its own "package"

Use case

This would let code in long-running services utilize host services if they wanted to, giving people more built-in goodness when they use Nex deployments.

Contribution

No response

[jordan-rash]

@kthomas should this be closed by #195??

[autodidaddict]

This is a broader scope than the cleanup. For example, the host services client isn't a thing that can be used by external code yet

[kthomas]

@kthomas should this be closed by #195??

#195 was needed as a prereq to formalizing an SDK, in that message headers and non-enveloped payloads were made consistent across the v8 implementation.

[autodidaddict]

Working on this now