Add ChildWorkflowOptions support to WorkflowImplementationOptions (#2)

[porunov]

Fixes #2790

Tests are added to: ChildWorkflowOptionsInWorkflowImplementationOptionsTest.java

• Add childWorkflowOptions map and defaultChildWorkflowOptions fields
• Add setChildWorkflowOptions() and setDefaultChildWorkflowOptions() builder methods
• Add getChildWorkflowOptions() and getDefaultChildWorkflowOptions() getters
• Update SyncWorkflowContext to store and expose child workflow options
• Update WorkflowInternal.newChildWorkflowStub() to merge predefined options
• Add mergeChildWorkflowOptions() method to ChildWorkflowOptions.Builder
• Add integration and unit tests

Potential documentation place: https://docs.temporal.io/develop/java/workflows/child-workflows (it could be something similar as activity options documentation).

Disclaimer: Code was Generated by Opus 4.6 in Copilot. Reviewed manually.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}