feat: Revendor flatcc

[bkietz]

After dvidelabs/flatcc#289 flatcc will not enforce alignment on empty vectors. This will let us unskip the null_trivial case in the integration tests

[paleolimbot]

Thanks!

I think it just need a cmake-format -i CMakeLists.txt and to possibly add flatcc_generated.h to the files we ignore for clang-format in .pre-commit.yaml.

[bkietz]

to possibly add flatcc_generated.h to the files we ignore

It is already excluded, and everything in thirdparty/ too. However the CI job uses --all-files which overrides exclusions. This seems to have been the case since pre-commit was added to the project, so I assume we want to remove that flag now

[paleolimbot]

--all-files which overrides exclusions

This is not true for me locally (e.g., if I remove thirdparty/ then all of flatcc becomes reformatted when I run pre-commit run --all-files), but other than this and ADBC I am not all that familiar with the details. We could also just reformat it as part of the vendoring if it causes too much trouble.