User Trigger

## Brief feature description

As a user one might want to trigger a `WaitSet` that is waiting in another thread. Currently, this would require an additional service which is only used for this intra-process trigger mechanisms which seems a bit like an overkill.

In classic iceoryx we had `UserTriggers` which we could reimplement in iceoryx2. They would be based on file descriptors, not inter-process capable and can be attached to a waitset.

iceoryx2 already has a `socketpair` abstraction we use for process local events. We could use the same base building block to realize this user trigger.

Start by reviewing the existing WaitSet and socketpair abstraction used for process-local events. Compare the proposed file-descriptor-based UserTriggers with classic iceoryx UserTriggers and determine how they would attach to a WaitSet and wake a waiting thread. Done means the scope and behavior of an intra-process UserTrigger are defined and implemented with appropriate coverage.

Written by the indexing model from the issue text.