Fix `setattr` usage on `FederationRateLimitConfig.__init__()`

[ShadowJonathan]

(Kinda successor of #8376, with #8351 handling the other bit of that issue.)

Doing setattr (via kwargs or otherwise) upon instance initialisation disallows static analysis of the types those variables have, or it's existence (properly).

FederationRateLimitConfig currently does this, though it's not used as a type annotation anywhere, I think this is a precursor to that.

synapse/synapse/config/ratelimiting.py

Lines 41 to 43 in 2ca4e34

	def __init__(self, **kwargs):
	for i in self._items_and_default.keys():
	setattr(self, i, kwargs.get(i) or self._items_and_default[i])

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(I saw that someone fixed this snippet i pointed at in the previous issue, thank you @ who ever fixed that ❤️)

[squahtx]

Looks like this has been addressed by #10601