Add MOQ5 as an opt-in publishing backend

[rwl4]

Route the batch, live stdin/SRT, and live-object publish paths through the moq5 service tier when -DOPENMOQ_USE_LIBMOQ_PUBLISHER=ON, alongside the existing picoquic transport. A compile-time gate keeps the default path unchanged while the integration matures; an injected TransportFactory always forces the legacy path regardless of the gate.

The moq5 path translates moqxr publish plans into media-sender objects with demand-aware subscription gating, bounded WOULD_BLOCK backpressure handling, CMSF SAP metadata propagation, and a graceful transport drain (moq_endpoint_drain) that flushes queued stream bytes before teardown so a slow or in-flight object is not truncated.

Package coalesced progressive-MP4 media as bounded, keyframe-aligned CMAF objects: one group per track with a per-GOP object, long GOPs split into capped continuation objects, rather than a single whole-track payload -- keeping each fragment's sample table within the receiver's CMAF validation limits while preserving group-start SAP types.

CMake gains sibling moq5/picoquic/picotls detection, OpenSSL pinning for the picotls build, and WebTransport adapter enablement. Adds translation and packaging tests and documents the backend selection in docs/build.md and the README.

[mondain]

Let me know your thoughts here; also for support of forward == 1, no subscriber is required for media to flow to a relay, which is a valid use-case when a relay implementation supports non-moq subscription or recording.