Kafka Connect: Split route field path once instead of per record

[wombatu-kun]

When routing by a field (static routing with iceberg.tables.route-field, or dynamic routing), SinkWriter extracted the route value for every record via RecordUtils.extractFromRecordValue(value, routeField), which re-parsed the dotted field path with Splitter.on('.').splitToList(routeField) on each call. The route field is fixed for the connector's lifetime, so this re-parse is pure per-record overhead.

This splits the path once in the SinkWriter constructor and adds a RecordUtils.extractFromRecordValue(Object, List<String>) overload that takes the already-split path; the existing String overload now delegates to it, so other callers are unchanged. Behavior is identical.

A throwaway A/B microbench over the whole extractFromRecordValue method (2M iterations x 9 trials, median; baseline = current String overload that splits per call, optimized = List overload with the path split once) showed:

record value	route field	before	after	faster
struct	key	52.8 ns	5.7 ns	89%
struct	data.id.key	162.2 ns	32.0 ns	80%
map	key	54.3 ns	6.1 ns	89%
map	data.id.key	144.3 ns	21.6 ns	85%

That is roughly 47 ns saved per record for a single-segment route field and ~120-130 ns for a three-segment path, paid once per record on the routing path. The numbers are indicative wall-clock from a microbench, not JMH.

Existing TestSinkWriter and TestRecordUtils cover both routing modes and the extraction overloads.