Port system Nexus envelope encoding rewrite

[tconley1428]

Summary

• port the Python SDK system Nexus envelope handling to the Go SDK
• keep the outer SignalWithStartWorkflowExecution system envelope unencoded while rewriting nested user payloads through codec and external storage
• add unit and integration coverage for the new rewrite path

Testing

• GOCACHE=/tmp/temporal-sdk-go-cache go test ./internal -run TestSystemNexusOutboundPayloadVisitor_RewritesNestedPayloadsOnly -count=1
• GOCACHE=/tmp/temporal-sdk-go-cache go test ./internal -run TestDoesNotExist
• GOCACHE=/tmp/temporal-sdk-go-cache go test ./temporalnexus/system -run TestDoesNotExist
• GOCACHE=/tmp/temporal-sdk-go-cache go test . -run TestSystemNexusDefersOuterEnvelopeEncoding -count=1

[tconley1428]

It isn't really a payload converter in all cases