Refactor span output to a common function

[RonFed]

Introduce the output_span_event function which is used by all probes to send spans to user space.
This has the following advantages:

• Spans that are not sampled will not be sent to user space, and the new function contains that check.
• The perf buffer is defined in a single place, similarly to what was done in user space in Refactor probe loading to use ebpf.Collection #899. This allows for future improvements such as using ring buffer instead of perf buffer if the kernel supports it (as suggested in Performance Testing & Benchmarking Go Auto Instrumentation #334 (comment)).