Avoid Option allocation in hook duration path via HookData.getOrNull

[EtaCassiopeia]

Summary

The structuredLogging.elapsed, metricsDetailed.after, and metricsDetailed.error hooks read the recorded start time from HookData to compute evaluation duration. The existing code used hookData.get(key).getOrElse(end) which allocates an Option on every flag evaluation — a hot path.

Adds HookData.getOrNull(key): Any (internal, private[openfeature]) that returns the raw value or null, and updates the three duration call sites to pattern-match on null. As a side benefit, when no start time was recorded (defensive case), the read of the end-time clock is skipped entirely and Duration.Zero is returned.

Clock.nanoTime is preserved (consistent with both before and tests), so TestClock interactions remain correct.

Changes

• Hook.scala: add HookData.getOrNull (internal API)
• Hook.scala: rewrite structuredLogging.elapsed, metricsDetailed.after, metricsDetailed.error to use null-first pattern match

@EtaCassiopeia

Credit

Spotted in #110 by @guizmaii. The original PR also switched Clock.nanoTime → System.nanoTime; this PR keeps Clock.nanoTime for testability.

[EtaCassiopeia]

Tested independently; closing in favor of #110.