[Feature Request] Put `temporal-namespace` header on all requests from pure JS client

[cretz]

Describe the solution you'd like

Pure JS client needs to have a gRPC interceptor that takes the namespace field out of the request object if present (do not try to use the namespace from client options), and if it is present, set the temporal-namespace header with that value.

Also include a CI test that only runs for non-forked PRs that runs a very simple workflow against cloud with API key (can see Ruby CI or ask for clarification and I (@cretz) would be happy to help clarify how).

See temporalio/features#475