Nexus operation testing environment

[cretz]

Describe the solution you'd like

Nexus operation authors, like activity authors, need an environment to test their Nexus operations. It's probably just the ability to mock and trigger certain situations without server involvement, much like activity test environments. This environment should probably be built in the non-Temporal Nexus SDKs then the Temporal SDKs can add features.

Per-SDK Tickets

• Go - TODO
• Nexus Go - TODO
• Java - TODO
• Nexus Java - TODO
• Core - N/A
• TypeScript - TODO
• Nexus TypeScript - TODO
• Python - TODO
• Nexus Python - TODO
• .NET - TODO
• Nexus .NET - TODO
• Ruby - TODO
• Nexus Ruby - TODO
• PHP - TODO
• Nexus PHP - TODO
• Temporal CLI - N/A