feat: migrate wasmedge_ffmpeg

[0yi0]

Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840). Blocked by the pilot migration (see Relationships) — follow the recipe in docs/MIGRATING.md once it lands.

Migrate wasmedge_ffmpeg to build, test, and ship from this repository.

• Enable wasmedge_ffmpeg in the standalone CMake build (its existing WASMEDGE_PLUGIN_* option) against the pinned WasmEdge SDK
• Port tests from test/plugins/wasmedge_ffmpeg and make them pass locally — or add a smoke test if no upstream suite exists
• Add CI following the pilot pattern: .github/workflows/build-wasmedge_ffmpeg.yml + .github/scripts/wasmedge_ffmpeg/
• Add the plugin row to README.md
• After the first release containing wasmedge_ffmpeg: tick it off in the core-removal tracking issue

Migration notes: needs FFmpeg dev libraries (avcodec/avformat/avutil/swscale/swresample); upstream tests exist.

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