GPU: Add GPUO2ExternalUser.h, to be included by external libraries using GPUTracking classes, instead of including the full GPUO2Interface.h + some fixes

[davidrohr]

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[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for eb34c5b at 2025-11-17 02:04:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/14824-slc9_x86-64/0/GPU/Workflow/src/GPUWorkflowITS.cxx:32:46: error: invalid use of incomplete type 'class o2::gpu::GPUO2Interface'
/sw/SOURCES/O2/14824-slc9_x86-64/0/GPU/Workflow/src/GPUWorkflowITS.cxx:33:80: error: invalid use of incomplete type 'class o2::gpu::GPUO2Interface'
/sw/SOURCES/O2/14824-slc9_x86-64/0/GPU/Workflow/src/GPUWorkflowITS.cxx:57:11: error: invalid use of incomplete type 'class o2::gpu::GPUO2Interface'
ninja: build stopped: subcommand failed.

Full log here.