wifi: PHY refactorization part 4: WifiMode refactoring

This MR on WifiPpdu refactoring is the 4th part of a series of 5 MRs on PHY refactorization. It is rebased on top of !519 (merged) so only the last 6 commits (i.e. starting from "wifi: Add ModulationLookupTable for WifiMode rework") are relevant for this MR.

This work was done by @kyuucr within the NSoC (mentored by Sébastien, Hany, and myself)