ITS::gpu: Add processNeighbours GPU kernel and handler

[mconcas]

After struggling in having this working in one PR, I split this development in two and will be pushing relative progress in different PRs:

• First iteration of the processNeighboursHandler is working, so it is progressing to the full offload.
• Second iteration is working as well, but requires still I/O from host, at the moment.

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
To request your PR to be included in production software, please add the corresponding labels called "async-" to your PR. Add the labels directly (if you have the permissions) or add a comment of the form (note that labels are separated by a ",")

+async-label <label1>, <label2>, !<label3> ...

This will add <label1> and <label2> and removes <label3>.

The following labels are available
async-2023-pbpb-apass4
async-2023-pp-apass4
async-2024-pp-apass1
async-2022-pp-apass7
async-2024-pp-cpass0
async-2024-PbPb-apass1
async-2024-ppRef-apass1

[alibuild]

Error while checking build/O2/fullCI for 219d92b at 2024-12-20 05:36:

## sw/BUILD/O2-latest/log
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
c++: error: unrecognized command-line option '--rtlib=compiler-rt'
/sw/SOURCES/O2/13822-slc8_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/Cell.h:18:10: fatal error: 'iostream' file not found
/sw/SOURCES/O2/13822-slc8_x86-64/0/Detectors/ITSMFT/ITS/tracking/include/ITStracking/Cell.h:18:10: fatal error: 'iostream' file not found
ninja: build stopped: subcommand failed.

Full log here.