ITS: GPU: partial Vertexer implementation

[f3sch]

Partial implementation of the Vertexer. The last step is missing, where I explicitly crash the workflow for now.

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 9defabd at 2025-09-13 11:07:

## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ GRERR=0
++ grep -v clang-diagnostic-error error-log.txt
++ grep ' error:'
error: error: clang-tidy: Unknown command line argument '--load /sw/slc9_x86-64/o2codechecker/v20.1.7-local2/lib/libclangTidyAliceO2Module.so --extra-arg=--gcc-install-dir='.  Try: '/sw/slc9_x86-64/Clang/v20.1.7-1/bin-safe/clang-tidy --help'
++ [[ 0 == 0 ]]
++ exit 1
--

Full log here.