feat: add trace.stdext, with standard extensions

src/stdext/dune

@@ -0,0 +1,5 @@
+(library
+ (name trace_stdext)
+ (public_name trace.stdext)
+ (synopsis "standard extensions for trace")
+ (libraries trace.core))

src/stdext/trace_stdext.ml

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+
+(** Standard extensions *)
+
+open Trace_core
+
+type extension_event +=
+  | Link of explicit_span * explicit_span_ctx
+  (** [Link (span, ctx)] establishes a link between two spans (one of which is
+  only known by context). The link is not a parent-child link, but a causality/relatedness link.
+  Each collector is free to interpret this as it wishes. *)
+
+(** Link two spans together. *)
+let[@inline] link (es:explicit_span) (ctx:explicit_span_ctx) : unit =
+  Trace_core.extension_event (Link (es, ctx))