Set Temporal-Namespace header on every namespace-specific gRPC request

[cretz]

Describe the solution you'd like

See temporalio/features#475. It needs to be an interceptor surely, but unsure how the implementation will do this generically. One way is via reflection to call getNamespace() string assuming it's not deemed too expensive (probably not), but this is left up to the implementer. All that matters is that new RPCs that come in with namespaces automatically have the header set.