[#16670] dist-trace: carry trace context across task and Raft-apply thread hops

[ellabaron-code]

Same capture/re-activate pattern as the async scope-carry infra, applied
to the higher-level task types whose work runs on a different thread than
the one that triggered it. Each snapshots dist_trace::GetActiveSpanContext()
where the scope is still live and re-establishes it with ActivateParentScope
around the deferred run, so the work's RPCs nest under the triggering trace.

• tablet/operation_driver: OperationDriver captures at construction,
  re-activates in ApplyTask (APPLYING/APPLIED UpdateTransaction RPCs on
  the Raft apply thread).
• master/async_rpc_tasks_base: RetryingRpcTask captures at construction,
  re-activates at the top of Run().
• master/multi_step_monitored_task: captures at construction, re-activates
  in RunInternal before each step.
• tserver/tablet_validator: captures per scheduled index (carried from the
  CreateTablet RPC into OpenTablet), re-activates the first batched index's
  context around the GetBackfillStatus poll.

Purely additive; all no-ops when the captured context is invalid.

[ellabaron-code]

Misfiled: reopening against yugabyte/yugabyte-db.