infra: group spark files into dev/spark/ (#2563)

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Follow up to #2540
Related to #1527

# Rationale for this change
Put all the files related to spark inside `dev/spark/`

## Are these changes tested?

## Are there any user-facing changes?

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Author: kevinjqliu

dev/docker-compose-integration.yml

@@ -18,7 +18,7 @@
 services:
   spark-iceberg:
     container_name: pyiceberg-spark
-    build: .
+    build: spark/
     networks:
       iceberg_net:
     depends_on: