MINOR: otel: added flt_otel_sample_eval and exposed flt_otel_sample_add_kv

MINOR: otel: added flt_otel_sample_eval and exposed flt_otel_sample_add_kv

Factored the sample evaluation logic out of flt_otel_sample_add() into a
new flt_otel_sample_eval() function that evaluates a sample definition
into an otelc_value.  Both the log-format path and the bare sample
expression path are handled, with a flag_native parameter controlling
native type preservation for single-expression samples.
flt_otel_sample_add() now calls flt_otel_sample_eval() and dispatches the
result.

Made flt_otel_sample_add_kv() non-static so callers outside util.c can
add key-value pairs directly to a bare flt_otel_scope_data_kv without
requiring the full flt_otel_scope_data structure.