ot::KeyManager::ExtractKek() method

A reference to a `Kek` to output the retrieved KEK.

Sets the `KeyMaterial` from a given Key. If the `KeyMaterial` currently stores a valid `KeyRef`, the `SetFrom()` call will ensure to delete the previous one before creating and using a new `KeyRef` associated with the new `Key`.

Sets the KEK.

Extracts the literal key from `KeyMaterial`

Updates MAC keys and key index.