parseJson implementation in VariantBuilder

[gaurav7261]

Describe the enhancement requested

Every consumer of parquet-variant currently has to implement their own
JSON-to-Variant parsing (Spark, Kafka Connect, Flink, etc.). This adds
native parseJson() static methods to VariantBuilder using Jackson
streaming parser, ported from Apache Spark's production-proven logic.

• Single-pass streaming (no intermediate JsonNode tree)
• Smallest integer type selection (byte/short/int/long)
• Decimal-first float handling, double fallback for scientific notation
• Overloads for String input and pre-positioned JsonParser

https://github.com/apache/spark/tree/master/common/variant

Component(s)

Core

[steveloughran]

#3451 relates to this; that's an issue/pr for benchmarking variant operations.