Access to workflow/activity instance created by Temporal from workflow/activity context where applicable

[cretz]

Describe the solution you'd like

For instances created by Temporal, they should be accessible from the context. Users want to access the actual class instance of a workflow from, say, an interceptor. This needs to be made available from the workflow context as the "instance". Note this is not applicable to Go and TypeScript. And for Java, PHP, Ruby, and .NET (DI only), the activity instance should be made available from the activity context (since they create the instance and control its lifetime).

• Go - N/A
• Java - Access to workflow/activity instance from context sdk-java#2361
• Core - N/A
• TypeScript - N/A: Workflows and Activities are simple functions, not classes
• Python - [Feature Request] Access to workflow instance from context sdk-python#720
• .NET - [Feature Request] Access to workflow/activity instance from context sdk-dotnet#388
• Ruby - [Feature Request] Access to workflow/activity instance from context sdk-ruby#180
• PHP - [Feature Request] Access to workflow/activity instance from context sdk-php#546
• Temporal CLI - N/A

[josh-berry]

Also need to do this for Nexus as well (for Java, and for other langs once ready).