Add borsh support for PeerMessage::DistanceVector

[saketh-are]

In #9187 we introduced the first new PeerMessage variant in a long time, called DistanceVector.

I got a little over-zealous about our plans to deprecate borsh and skipped implementing borsh support for the new message variant.

However, it turns out we have some test infrastructure still reliant on borsh-encoded connections:

nearcore/pytest/lib/proxy.py

Lines 89 to 90 in 6cdee7c

	# TODO(CP-85): when removing borsh support, fix this to use protobufs,
	# (or preferably reimplement the test in rust).

In particular, the nayduck test pytest/tests/sanity/sync_chunks_from_archival.py makes use of the proxy tool and is failing after #9187.

This PR implements borsh support for DistanceVector as an immediate fix for the failing test.

In the long run we aim to deprecate borsh entirely, at which time this code (and a bunch of other code much like it) will be removed.

[saketh-are]

As a sidenote, there is a different PeerMessage variant called Tier1Handshake which does not have borsh support implemented:

nearcore/chain/network/src/network_protocol/borsh_conv.rs

Lines 168 to 170 in 6cdee7c

	mem::PeerMessage::Tier1Handshake(_) => {
	panic!("Tier1Handshake is not supported in Borsh encoding")
	}

It does not pose the same issue because Tier1Handshake is only sent over TIER1 connections, and there is some special handling which forces TIER1 connections to use protobuf encoding (and not borsh):

nearcore/chain/network/src/peer/peer_actor.rs

Lines 280 to 281 in 6cdee7c

	// Override force_encoding for outbound Tier1 connections,
	// since Tier1Handshake is supported only with proto encoding.