RootCanal: Implement extended advertising PDUs

Refactor LE extended advertising to implement HCI parameter
validation as required by the specification.

Add a new link layer packet type LE_EXTENDED_ADVERTISING_PDU
corresponding to ADV_EXT_IND, AUX_ADV_IND, AUX_CHAIN_IND PDUs,
and implement proper handling for extended scanning and legacy
initiator states

Test: atest --host rootcanal_hci_test
Test: gd/cert
Test: pts-bot
Change-Id: I6696ccc47de7b72715e6f6eeda3ab4c5511e11af