[Reset] Add children_initialized_post_reset_point to WorkflowExecutionInfo for tracking child workflows post-reset (temporalio#7194)

## What changed?
Added a new field `children_initialized_post_reset_point` to collect a
set of children that need to be restarted after the parent is reset.

## Why?
We need to remember the set of children that need to be restarted after
resetting a parent. So adding this field which will be populated in
`workflowResetterImpl` (in the followup PR)

## How did you test it?
Existing tests. This is just the protochange.

## Potential risks
N/A

## Documentation
N/A

## Is hotfix candidate?
No

Author: gow