Implement API for reading extended data by plugins

[nosoop]

In the future I'd like to implement the following native functionality:

/**
 * Returns a subsection within an items "extended data" section.  This handle should be freed by the calling plugin.
 * If the section doesn't exist, this returns `null`.
 */
native KeyValues CWX_GetItemExtData(const char[] uid, const char[] name);

// TODO some item UID iterator (??)

One of the intended functions of this setup is to allow custom-defined sections, such as for precaching resources on map startup, and just things that other plugins can hook on:

"my item"
{
	"name"		"My custom bat that I didn't steal from Smash or anything"
	"inherits"	"TF_WEAPON_BAT"
	"extdata"
	{
		"resources"
		{
			"model"		"models/path/to/a/custom_bat.mdl"
			"texture"	"materials/stuff.vmt"
			"texture"	"materials/more_stuff.vmt"
			"sound"		"sound/satisfying_bat_impact_sound.wav"
		}
	}
}

I believe this is something that would be relevant for #7.

(The reason we do it this way is to isolate plugin-specific data and provide a convenient interface without having to traverse deep into the entire item KeyValues struct. Also, we may not necessarily require items to be defined in their entirety using KeyValues.)

[nosoop]

Closed with 72aadbf for the most part.