Add move-friendly buffer accessors

[alienx5499]

Overview

Implements the move-friendly accessor improvements requested in issue #95. Adds ref-qualified and rvalue overloads so temporaries can return buffers by move, eliminating unnecessary copies while keeping lvalue behavior unchanged.

Problem

• toBuffer/toVector returned const references only, forcing an extra copy when called on temporaries.
• Multihash stored data in a const shared_ptr, blocking safe move-out even when uniquely owned.

Solution

• Add const&/&& overloads for toBuffer and toVector on Multihash, UVarint, and PeerId.
• Allow Multihash to move out its byte buffer when uniquely owned; fall back to const& otherwise.
• Preserve existing API surface for lvalues; only move semantics change for rvalues.

Key Changes

• include/libp2p/multi/multihash.hpp / src/multi/multihash.cpp: ref-qualified accessors; move-out path when Data is uniquely owned.
• include/libp2p/multi/uvarint.hpp / src/multi/uvarint.cpp: add rvalue toVector to move backing bytes from temporaries.
• include/libp2p/peer/peer_id.hpp / src/peer/peer_id.cpp: rvalue toVector delegates to moved Multihash.

Closes #95