Add root workflow execution to WorkflowExecutionStartedEventAttributes

[rodrigozhou]

What changed?
Add fields for root workflow execution.

The root workflow execution is defined as follows:

1. A workflow without parent workflow is its own root workflow.
2. A workflow that has a parent workflow has the same root workflow as its parent workflow.

Examples:

• Scenario 1: Workflow W1 starts child workflow W2, and W2 starts child workflow W3.
  • The root workflow of all three workflows is W1.
• Scenario 2: Workflow W1 starts child workflow W2, and W2 continued as new W3.
  • The root workflow of all three workflows is W1.
• Scenario 3: Workflow W1 continued as W2.
  • The root workflow of W1 is W1 and the root workflow of W2 is W2.

Why?

Breaking changes

[dandavison]

Could you add some documentation to the code explaining what the fields are and how they should be used? (Also the PR itself doesn't have any explanation, but that could come from the code comments in a proto API PR).

[cretz]

Approving from API POV, but would like to see reset behavior on this field documented (i.e. that it doesn't change across reset if that's the behavior)

[dandavison]

Thanks for explaining! I added an optional suggestion to make it explicit that if A--CAN-->B then parent(B) = parent(A).

[rodrigozhou]

The implementation in the Server is here: temporalio/temporal#5520