Add H2 Interval converters for Duration

[yeomin4242]

Closes spring-projects/spring-data-r2dbc#502

Add H2 Interval to Duration conversion support to the R2DBC H2 dialect.

This follows the feedback from #2248 by using the H2 API directly instead of reflection, while guarding registration by classpath presence.

Changes include:

• registering org.h2.api.Interval as a dialect simple type when H2 is available
• adding Interval -> Duration and Duration -> Interval converters
• supporting H2 day-time intervals and rejecting year-month intervals that cannot be represented as Duration
• adding unit and integration tests for simple type registration, conversion, writing, and H2 round-tripping

com.h2database:h2 is added as an optional dependency because the converters directly use org.h2.api.Interval, which is provided by the H2 artifact.

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