Bluetooth: controller: Scan Request Notifications

Implement the framework for LE Scan Request Received Event.
The feature is available under the Controller's advanced
features and will be selected implcitly when Bluetooth v5.0
LE Advertising Extensions feature is implemented.

Jira: ZEP-2073

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>