NlriEncoding::MPLS - Parse label (and reserved/BoS)

[thepacketgeek]

Currently the MPLS parsing in nlri.rs skips the label (and reserved/bottom-of-stack) bytes:

bgp-rs/src/update/nlri.rs

Lines 71 to 72 in fa8cafc

	let len_bytes = (f32::from(len_bits) / 8.0).ceil() as u8;
	// discard label, resv and s-bit for now

Support label parsing, along with multiple labels support