Phase 15: Engine Client Migration

[bvaradar]

Replace method calls in engine-specific clients (Spark, Flink, Java) from Avro Schema to HoodieSchema.

Scope: hudi-client, hudi-spark-datasource, hudi-flink-datasource packages

Changes: Update all client code to use HoodieSchema methods, replace schema parameter types, update engine-specific integrations

Strategy: Engine-by-engine migration with isolated testing, coordinate with engine compatibility requirements

Constraints: Maintain engine API compatibility, preserve existing performance characteristics, keep record formats unchanged

[rahil-c]

@the-other-tim-brown @voonhous I can take this

[the-other-tim-brown]

I'll start on the flink migration for this ticket and Rahil can take up the spark side changes