[WIP] Add experimental Event Groups support

[mjameswh]

Summary

Add experimental support for Event Groups.

Event Groups is a new form of Workflow-level metadata that allows for improved visibility into a Workflow execution's history by grouping logically related Events together based on user-defined or system-inferred criteria.

• Explicit groups: Workflow.create_event_group(id, label: "...") takes the Event Group's ID as its first and only required argument. The user-provided ID is used verbatim as the grouping key and should not contain sensitive information. The label is optional and, if provided, is a codec-encoded Payload. Attach a group per call (event_groups: [...]) or ambiently with Workflow.with_event_groups(...) { } (propagated through fibers). Nested scopes compose; markers dedupe by id.
• Implicit groups: automatically wrap signal and update handlers.
• Markers are serialized onto outgoing commands as event_group_markers. The user-facing type is EventGroup; the protocol type remains EventGroupMarker.
• Exposes Workflow::Info#original_execution_run_id (plumbed core → lang) and bumps the sdk-core submodule to the matching build.

No child-handle cancel (EG-COMMANDS-07-CHILD): ChildWorkflowHandle has no cancel API.

Testing

Event Groups support requires an Event-Groups-capable server. Local tests pin Temporal CLI:

v1.8.3-server-1.32.0-162.0

bundle exec rake test TEST=test/worker_workflow_event_groups_test.rb

Cloud environments skip history-transcription cases.