Adds API for resynchronizing sleepy child with its parent

[elie-elkhoury]

PLEASE NOTE: This PR is related to the following two other PRs, and in order for CI to pass on all the PRs, they should be merged in this order:

1. This PR here
2. Cascoda/knx_iot_code_gen#608
3. Cascoda/knx_iot_cascoda#138

New function otThreadSleepyChildResynchronize(), which will essentially send an MLE Child Update Request. Use this for child/parent resynchronization purposes, as described in the Thread spec Thread1.1.1 section 4.7.5, Timing Out Children:

Rx-off-when-idle Children SHOULD also periodically (at time intervals indicated by the application) resynchronize by
sending a Child Update Request to their Parent to avoid cases when the MAC Data Request
is being acknowledged erroneously by a receiver that is not a valid Parent.