Support custom failure workflow version in workflow termination

## Summary

When a workflow fails and triggers a failure workflow, there is no way to specify which **version** of the failure workflow to use. The current `WorkflowDef` only has a `failureWorkflow` string field (name), with no version parameter.

## Desired Behavior

- Add `failureWorkflowVersion` field to `WorkflowDef`
- `terminateWorkflow()` in `WorkflowExecutorOps` should use the specified version when starting the failure workflow
- If no version is specified, maintain current behavior (use latest version)

## Affected Files

- `common/src/main/java/com/netflix/conductor/common/metadata/workflow/WorkflowDef.java`
- `core/src/main/java/com/netflix/conductor/core/execution/WorkflowExecutorOps.java`
- `core/src/main/java/com/netflix/conductor/core/execution/tasks/SubWorkflow.java`
- gRPC proto for `WorkflowDef`
- Documentation in `docs/documentation/configuration/workflowdef/`

## Prior Art

Community PR #696 by @phgbecker implemented this with tests, docs, and gRPC support (187 additions).