feat: add coderd_organization_sync_settings resource

[aslilac]

Part of #150

Technically, this is complete by itself, as you can entirely configure organization sync this way, but this PR does not include the proposed "partial" API that we would still like to implement.

Example:

resource "coderd_organization_sync_settings" "test" {
	field          = "wibble"
	assign_default = false

	mapping = {
		wobble = [
			"bf37c14a-cd36-4bf0-8ffb-e70d6ef8c518",
		]
	}
}

[ethanndickson]

I only have so much of an understanding of IdP sync settings, but this mostly lgtm!
My only real comments are on how this resource handles getting imported.

[aslilac]

I'm realizing that you don't actually have to implement import, and I think skipping it is better than a janky import that's weird and has potential issues/adds complexity.

There can only ever be one of this type of resource (which does break the Terraform model a bit, but is also what our customers want) so importing isn't actually very helpful here.