Add vendored Flatbuffers (v25.9.23) #222

Job	Run time
identifiers / Build GitHub identifiers	8s
identifiers / Log resolved identifiers	3s
Build wheels on macos-15 (arm64)	1m 59s
Build wheels on ubuntu-24.04 (x86_64)	1m 41s
Build wheels on windows-2022 (x86_64)	3m 39s
	7m 30s

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