[core] Load packet rate limits from config

[sruon]

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What does this pull request do?

Been meaning to fix this for a while.

Tweaked settings code to support nested tables and moved the existing rate limits to network.lua

Steps to test these changes

[Xaver-DaRed]

Is this how we call settings from core?
settings::get<data_type>("network.PACKET_RATE_WHATEVER");

[sruon]

settings::get cannot deal with nested/complex tables as it's coded to only hold primitive data types, hence why we're reaching into the state directly (this is also done in xi_search for the white list).

I do intend to leverage the YAML parser for reading and loading settings in the future which will get us rid of these hacks.

Changing settings::get to support complex nesting is more hassle than it's worth right now.