Make eager activity reservation limit configurable

[Sushisource]

What changed

• Replace Core's hardcoded limit of three eager activity slot reservations per workflow-task completion with max_eager_activity_reservations_per_workflow_task.
• Preserve the existing default of three; setting the option to zero disables eager activity reservations.
• Expose the same option through the Rust SDK and C bridge.
• Cover configured reservation behavior and option propagation.

Why

Workers already cap the number of slots they attempt to reserve for eager activities while completing a workflow task. Making that cap configurable lets applications tune the reservation burst without introducing a separate execution-lifetime limit.

Validation

• cargo test -p temporalio-sdk-core activity_tasks_from_completion_are_delivered
• cargo test -p temporalio-sdk max_eager_activity_reservations_per_workflow_task_propagates
• cargo check -p temporalio-sdk-core-c-bridge
• cargo lint
• cargo test-lint

No breaking changes; the default remains three.