Support span links in the OpenTelemetry reporter

[linhr]

First of all, thanks for creating this awesome project!

OpenTelemetry supports span links to declare relations among spans from different traces. Currently, in OpenTelemetryReporter, the links field of OTel SpanData is set as SpanLinks::default(). It would be a nice feature if fastrace could provide methods to record span links, and export the information to other OTel collectors.