MINOR: otel: added span link support

MINOR: otel: added span link support

Added span link support, allowing a span to reference other spans or
extracted contexts without establishing a parent relationship.

Introduced the flt_otel_conf_link structure and added a links list to
flt_otel_conf_span.  The parser accepted both an inline syntax on the span
declaration line ("span <name> link <target>") and a standalone multi-
argument form ("link <span> ..."), each creating a conf_link entry
appended to the span's link list.

At runtime, each configured link name was resolved against the active
spans and extracted contexts in the runtime context.  Resolved references
were collected into flt_otel_scope_data_link entries and passed to the C
wrapper add_link API during span creation.

Initialization, cleanup, and debug dump routines were added for the link
data structures at both configuration and runtime levels.