SPINEL_PROP_THREAD_MGMT_GET_PENDING_DATASET value

Send MGMT_GET Thread Pending Operational Dataset Format: `A(t(iD))` - Write only The formatting of this property follows the same rules as in SPINEL_PROP_THREAD_MGMT_GET_ACTIVE_DATASET. This is write-only property. When written, it triggers a MGMT_PENDING_GET meshcop command to be sent to leader with the given Dataset. The spinel frame response should be a `LAST_STATUS` with the status of the transmission of MGMT_PENDING_GET command.