Add vendored Flatbuffers (v25.9.23) #223

Job	Run time
identifiers / Build GitHub identifiers	8s
identifiers / Log resolved identifiers	2s
Build wheels on macos-15 (arm64)	3m 15s
Build wheels on windows-2022 (x86_64)	6m 18s
Build wheels on ubuntu-24.04 (x86_64)	2m 0s
	11m 43s

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