Draft: wifi: Refactor information elements to facilitate future extensions

As agreed offline with Tom and Doug, I am opening this MR (this is still a draft, e.g. it is not rebased on a recent master and it is not split in smaller commits yet, …) in order to find a consensus about how we should handle IEs while keeping in mind both 801.11be and future extensions as well as pluggable module like 802.11ad/ay or 802.11ah (and maybe few more in the future). This is based on Tom’s proposal described in #173 (closed)

We are currently observing an increasing divergence between what we are doing for wigig and what is being done for 11be, and this solution used in wigig can no longer be rebased now that some multi link changes have been merged. If more optimal, I suggest to organise a meeting among people involved in wigig and 11be (mostly @stavallo, @tomhenderson and @sderonne) to discuss about this MR.