Define SpanAttachedEvent query protocol

[wu-sheng]

# SpanAttachedEvent represents an attached event for a traced RPC.
# When an RPC is being traced by the in-process language agent, a span would be reported by the client-side agent.
# And the rover would be aware of this RPC due to the existing tracing header.
# Then, the rover agent collects extra information from the OS level to provide assistance information to diagnose network performance.
#
# Notice, THIS IS ALSO AVAILABLE FOR ZIPKIN SPAN.
# -----------------------------------------------
# In SkyWalking, ZipkinQueryHandler provides full support for all Zipkin span queries.
# SpanAttachedEvent query is supported through the trace query URI: /api/v2/trace/{traceId}
# A new `attachedEvents` field would be added in JSONArray format with SpanAttachedEvent in JSON as elements.

The SpanAttachedEvent would be added to the current trace query automatically, both for SkyWalking and Zipkin

@Fine0830 Our trace query would adopt this part of the span, after @mrproliu implements the first SpanAttachedEvent report.

FYI @apache/skywalking-committers