[spark] Use jackson shaded by Paimon instead of Spark's built-in one

[kerwin-zk]

Purpose

Fix the todo left in paimon-spark/pom.xml by #6082:

<!-- todo(fix me): we should use the jackson libraries shaded by Paimon. -->

JsonUtils in paimon-spark-common was the only Scala code still using the unshaded jackson resolved from Spark's runtime (the spotless import-replacement rule that rewrites jackson imports to the shaded ones only applies to Java sources). So the jackson version it ran on was decided entirely by the Spark version (2.15.x for Spark 3.5, 2.18.x for Spark 4.x), which may differ from the version Paimon expects.

This PR:

• Rewrites PaimonSourceOffset json serde (the only user of JsonUtils) with JsonSerdeUtil.OBJECT_MAPPER_INSTANCE, i.e. the jackson shaded by Paimon. ScalaObjectMapper/DefaultScalaModule cannot be kept because paimon-shade-jackson-2 does not bundle jackson-module-scala; since the offset only has 3 primitive fields, the serde is written manually. The JSON field names and layout stay exactly the same, so streaming checkpoints remain compatible in both directions.
• Removes JsonUtils.
• Replaces the todo comment: the com.fasterxml.jackson.core:* exclusion on paimon-format itself is correct (paimon-format already shades jackson into its jar) and is kept.

Tests

Covered by existing PaimonSourceTest, which asserts offset json round-trips through query.lastProgress.sources(0).endOffset.

[JingsongLi]

+1