Purpose

Linked issue: Part of #3289

This PR adds the foundational infrastructure for FIP-37 RoaringBitmap SQL function implementation. It provides the serialization utilities, custom Flink type serializer, and base aggregate function class that will be used by the bitmap SQL functions (rb_build_agg, rb_or_agg, rb_and_agg, etc.) in subsequent PRs.

Brief change log

Added the following infrastructure files in fluss-flink/fluss-flink-common:

• BitmapUtils.java: Utility methods for serializing/deserializing RoaringBitmap using the ByteBuffer-based approach, which matches the server-side RoaringBitmapUtils.serializeRoaringBitmap32 format used by FieldRoaringBitmap32Agg for wire compatibility.
• RoaringBitmapSerializer.java: Custom Flink TypeSerializer for RoaringBitmap accumulators to ensure correct checkpoint/savepoint behavior. Without this, Flink falls back to Kryo which is sensitive to internal class layout changes across RoaringBitmap library versions.
• RoaringBitmapTypeInfo.java: TypeInformation wrapper that provides the custom serializer to Flink's type system.
• AbstractRbAggFunction.java: Base class for bitmap aggregate UDFs with @FunctionHint(accumulator = @DataTypeHint(value = "RAW", bridgedTo = RoaringBitmap.class)) annotation. This tells Flink's Table planner to skip reflection-based POJO field extraction on RoaringBitmap and use the custom TypeInformation instead.
• BitmapUtilsTest.java: Unit tests covering null handling, empty bitmap, known values round-trip, large cardinality (100K elements), and server serialization compatibility.
• RoaringBitmapSerializerTest.java: Unit tests covering RoaringBitmapSerializer and RoaringBitmapTypeInfo behavior, including serialization round-trip, copy methods, snapshot configuration, and type information checks.
• fluss-test-coverage/pom.xml: Added a JaCoCo exclusion for AbstractRbAggFunction, since it is abstract and cannot be directly covered by unit tests.
• pom.xml: Added RoaringBitmap dependency (version 1.3.0 from root pom).

The aggregate functions (rb_build_agg, rb_or_agg, rb_and_agg) and catalog registration will follow in subsequent PRs linked to this issue.

Tests

Unit tests added and passing:

• BitmapUtilsTest.testNullInputToBytes() - null handling.
• BitmapUtilsTest.testNullInputFromBytes() - null handling.
• BitmapUtilsTest.testEmptyBitmapRoundTrip() - empty bitmap serialization.
• BitmapUtilsTest.testKnownValuesRoundTrip() - correctness with known values.
• BitmapUtilsTest.testLargeCardinality() - performance with 100K elements.
• BitmapUtilsTest.testFormatCompatibleWithServerSerialization() - wire compatibility.

Tests are still running and this PR description will be updated with the final verification results once the full validation completes.

Verified with:

• ./mvnw spotless:apply -pl fluss-flink/fluss-flink-common - BUILD SUCCESS.
• ./mvnw test -pl fluss-flink/fluss-flink-common -Dtest=BitmapUtilsTest - BUILD SUCCESS.
• ./mvnw clean install -pl fluss-flink/fluss-flink-common -DskipTests - BUILD SUCCESS.
• ./mvnw clean package -DskipTests (full project build) - BUILD SUCCESS.
• Checkstyle: 0 violations.

API and Format

This change does not affect any public API or storage format. It adds internal infrastructure utilities that will be used by future bitmap SQL functions.

Documentation

This change does not introduce new user-facing features yet. The bitmap SQL functions (rb_build_agg, rb_or_agg, rb_and_agg) and their documentation will be added in follow-up PRs.