Add support for forwarding Nexus HTTP requests

[pdoerner]

What changed?

• Added a new component cluster.HttpClientCache which serves a similar purpose to our gRPC ClientBean but provides HTTP clients for remote clusters.
• Added logic to forward Nexus requests from standby clusters to active.
• Exposed frontend namespace.Registry from our test temporalImpl so that tests can use Eventually functions to wait for namespace data to be updated in-memory without having to use time.Sleep

Why?

So that Nexus requests can be forwarded across clusters.

How did you test it?

New unit and functional tests.