Fix Beyla CTRL+C shutdown. Also simplify pipeline

[mariomac]

In preparation for task #249 . The change is not big but it affects multiple files, as it has changed an ubiquitous type.

First, we have movedtransform.HTTPRequestSpan to request.Span in its own package.

Before this PR, eBPF tracers could send either []HTTPRequestTrace or []HTTPInfo slices. That mean that the Pipeline input channel had to be of the type []any and internally added a Codec that transformed each object to HTTPRequestSpan (In the Pipes library jargon, a codec is an component that automagically is attached to transform data when two connected pipeline stages do not match the output->input types).

Now the codec has been removed and the input channel is of the []request.Span instead of the []any type. Now each Tracer implementor has the responsibility to convert its input type to request.Span type, so the pipeline stages don't need to know about the eBPF types.

This will facilitate inserting the Service Name from the tracers, as we have now a unified type. It also will slightly improve speed, as we have removed a goroutine continuously iterating a channel.

This PR also fixes the sigint/sigterm handling, allowing Beyla to stop again without requiring a sigkill (bug introduced in PR #243).

[mariomac]

Not removed but moved to another file.

[grcevski]

LGTM! Looks much cleaner now, thanks! I think we might need to change this again in the future if we ever add different event types, e.g. security log events, but we'll cross that bridge when we get there :)

[mariomac]

@grcevski yes! I agree that at some point we'd need to expand Beyla for more event types. Another option is not make each event type to pass across the same pipeline but have their own pipeline, as there would be some nodes that wouldn't be useful in that case (for example, the log events maybe don't need the routes decoration node).