Fix/create ebpf/userland types

## Description

While working on the `proc` package, I've encountered an issue again regarding the appropriate types to use for specific fields across Tracee (both in BPF and userspace).

To summarize, focusing on the `pid` field. In BPF, we retrieve it using `bpf_get_current_pid_tgid`:
https://elixir.bootlin.com/linux/v6.12.6/source/kernel/bpf/helpers.c#L222

We typically extract `pid` and `tgid` as u32 each, which is a misconception induced by the helper function.

In reality, both are defined as `pid_t`:
https://elixir.bootlin.com/linux/v6.12.6/source/include/linux/sched.h#L1018

`pid_t` is, in turn, defined as `__kernel_pid_t`:
https://elixir.bootlin.com/linux/v6.12.6/source/include/linux/types.h#L27

which is an `int` or `s32`:
https://elixir.bootlin.com/linux/v6.12.6/source/include/uapi/asm-generic/posix_types.h#L28.

This means we've been treating them as unsigned when they should be signed, or as `int` in Go which has an unnecessary `int64` footprint. Beyond interpreting values incorrectly, this also leads to unnecessary conversions throughout the code.

The kernel itself makes use of `pid_t` as -1:

https://elixir.bootlin.com/linux/v6.12.6/source/kernel/exit.c#L1832
https://elixir.bootlin.com/linux/v6.12.6/source/kernel/pid_namespace.c#L222
https://elixir.bootlin.com/linux/v6.12.6/source/kernel/trace/trace_functions_graph.c#L1333
...

## Additional details

Related:

https://github.com/aquasecurity/tracee/issues/4504
https://github.com/aquasecurity/tracee/issues/3690
https://github.com/aquasecurity/tracee/pull/4484#discussion_r1925860171
https://github.com/aquasecurity/tracee/pull/4353