feat(aggregate): add Java-compatible field aggregators

[liangjie3138]

Purpose

Linked issue: close alibaba/paimon-cpp#457

Port the six practical field aggregators missing on the C++ side — collect,
merge_map, nested_update, rbm64, hll_sketch, theta_sketch .

Also adds GenericArray / GenericMap, the in-memory InternalArray /
InternalMap implementations these aggregators need to build ARRAY and MAP
results.

Tests

18 new unit test cases in paimon-core-test, covering each aggregator's
aggregate / retract / null / type-validation paths.

Three pin behaviour that is easy to "fix" incorrectly, and each was checked by
reverting the corresponding production change and confirming it fails:

• BinaryAggMergeFunctionTest.OwnedAccumulatorSurvivesNullInput — drives a real
  AggregateMergeFunction so the use-after-free is exercised through the
  production path.
• FieldNestedUpdateAggTest.CountLimitCountsNullElementsOfAccumulator — count
  limit is measured against the raw element count, matching Java.
• FieldIgnoreRetractAggTest.ReversedAggBypassesWrappedOverride — the wrapper
  does not preserve the wrapped aggregator's reversed-aggregation override, also
  matching Java.

No integration tests added.

API and Format

Additive only, no signatures changed:

• include/paimon/defs.h: four new Options constants — NESTED_KEY,
  NESTED_KEY_NULL_STRATEGY, NESTED_SEQUENCE_FIELD, COUNT_LIMIT.
• CoreOptions: a NestedKeyNullStrategy enum plus accessors.
• FieldAggregator: new virtuals default to the existing Agg / AggReversed.

New dependency: Apache DataSketches 5.2.0 (Apache-2.0), resolved through
resolve_dependency() like the other bundled dependencies.

Documentation

None.

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