Provide "workflow local"

[cretz]

Describe the solution you'd like

Users need a workflow-local var that is scoped only to the current workflow when accessed via code.

Rough sketch:

• A user can have var myValue workflow.LocalVar[string]
• myValue.Get(ctx) returns the workflow-specific value (zero value if never set, no need for a TryGet, a user can choose to wrap in nil-able type for optional/unset if needed)
• myValue.Set(ctx, value) sets the workflow-specific value

This is basically just type-safe sugar for top-level context value. See temporalio/features#571.