Add an immutable property.Value type for the Go SDK

Add an immutable property.Value type for the Go SDK: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). Add an immutable property.Value type for the Go SDK We want a clean, self-contained representation of a Pulumi property value that we can grow into a replacement for the…