fix(client): Properly set temporal-namespace header on gRPC requests

[mjameswh]

What was changed

• Temporal Cloud requires presence of the temporal-namespace gRPC metadata on every request to the Workflow Service that contains a namespace property. This was not done on the pure-JS Connection. There it is. Fixes [Feature Request] Put temporal-namespace header on all requests from pure JS client #1697.

• Note that this logic is taken care by Core SDK on NativeConnection.

• Also added CI tests against Temporal Cloud. Credentials are provided through environment variables. Tests will be skipped if credentials are missing.

Breaking notice

• IMPORTANT: We previously recommend Temporal Cloud users using API Keys to manually add the temporal-namespace metadata to their Client's connection config. This is no longer appropriate and must be removed. Keeping that metadata will result in connection failure, as the GetSystemInfo request must never contain a temporal-namespace metadata.

  Given that 1) API Keys were still marked @experimental in previous release, and 2) non-Temporal Cloud users may also need to add a temporal-namespace header that would have different constraints, we do not make any attempt to "automatically" remove that metadata is explicitly set on requests where it should not be present according to Temporal Cloud's expectations.