Expose instance to workflows/activities and client to activities

[cretz]

What was changed

• Add Temporalio.Workflows.Workflow.Instance to give access to the literal instance of the workflow class
• Add Temporalio.Extensions.Hosting.ActivityScope for activity scope information
  • This DI extension the only place .NET creates the user's instance for them
  • Provides both the ServiceScope and the ScopedInstance used by DI
  • Has advanced used for primary-execute-activity interceptor users to make their own scope or instance if needed since the instance is not available until inside the root call of the interceptor
• Add Temporalio.Activities.ActivityExecutionContext.TemporalClient to give access to Temporal client
  • Users can make test instances without this, which will throw on access
  • Users can technically make workers with a more generic IWorkerClient interface that is not the full client, which will throw on access
• Minor test refactoring and fix for tests since new dev server release

Checklist

1. Closes [Feature Request] Access to workflow/activity instance from context #388
2. Closes [Feature Request] Provide client from activity context #389