relay-hops: add hop path wire and negotiation support

[mondain]

Summary

• define the RELAY_HOPS, HOP_PATH, and EXCLUDE_HOP extension identifiers and message scopes
• encode and validate hop paths for MOQT drafts 16, 17, and 18
• negotiate relay-hops support independently on each session
• extend negotiated NAMESPACE messages with parameters while preserving the legacy wire format
• carry complete namespace parameters through publisher handles and allow specialized clients and servers to add setup options

Why

moqx needs the wire and session primitives from draft-lcurley-moq-relay-hops to propagate namespace paths, suppress returning advertisements, and safely support relay cycles.

Impact

The extension remains inactive unless both peers advertise the zero-length setup option. Sessions that do not negotiate it retain their existing namespace framing and behavior.

Validation

• MoQTests --gtest_filter='*RelayHop*:*ExcludeHop*' - 16 passed
• MoQSessionTests --gtest_filter='*RelayHops*:*NamespacePreservesRelayHopParameters*' - 15 passed
• git diff --check origin/main..HEAD

The corresponding moqx relay-policy PR will pin the final commit from this branch.

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