Make Dot11EltVendorSpecific packet extensible

[Hoolean]

See #4659

This commit allows the Dot11EltVendorSpecific packet to dispatch to its subclasses based on their OUI, with the same idiom that the parent Dot11Elt uses to select subclasses based on their ID already:

scapy/scapy/layers/dot11.py

Lines 1057 to 1074 in c15a670

	registered_ies = {}
	@classmethod
	def register_variant(cls, id=None):
	id = id or cls.ID.default
	if id not in cls.registered_ies:
	cls.registered_ies[id] = cls
	@classmethod
	def dispatch_hook(cls, _pkt=None, *args, **kargs):
	if _pkt:
	_id = ord(_pkt[:1])
	idcls = cls.registered_ies.get(_id, cls)
	if idcls.dispatch_hook != cls.dispatch_hook:
	# Vendor has its own dispatch_hook
	return idcls.dispatch_hook(_pkt=_pkt, *args, **kargs)
	cls = idcls
	return cls

This is just a rough sketch, so feedback very welcome; is the approach heading in the right direction? :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.57%. Comparing base (c15a670) to head (05b4154).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4660      +/-   ##
==========================================
- Coverage   81.55%   78.57%   -2.99%     
==========================================
  Files         359      334      -25     
  Lines       86557    81372    -5185     
==========================================
- Hits        70592    63934    -6658     
- Misses      15965    17438    +1473     

Files with missing lines	Coverage Δ
scapy/layers/dot11.py	91.44% <100.00%> (+0.20%)	⬆️

... and 286 files with indirect coverage changes