feat: add BTree global index core components

[lszskye]

feat: add BTree global index core components

Changes

BtreeDefs

• Constants struct defining BTree index configuration keys and default values: compression algorithm, compression level, block size, cache size, high-priority pool ratio, and read buffer size. Also defines the "btree" identifier.

BTreeFileFooter

• Footer structure for BTree index files containing bloom filter handle, index block handle, and null bitmap handle. Supports versioned serialization/deserialization via Read()/Write() with a magic number (0x50425449) for validation. Fixed encoding length of 52 bytes.

BTreeFileMetaSelector

• Implements FunctionVisitor<std::vector<GlobalIndexIOMeta>> to select candidate BTree index files based on filter predicates.

BTreeIndexMeta

• Per-file metadata holding serialized first key, last key, and a has_nulls flag. Keys can be null when the entire file contains only null values (OnlyNulls()). Provides Serialize()/Deserialize() for binary persistence with a 9-byte fixed header (two 4-byte key lengths + 1-byte null flag).

KeySerializer

• Static utility for serializing Literal values to Bytes and deserializing MemorySlice back to Literal, supporting all Paimon data types. Provides CreateComparator() to build a MemorySlice::SliceComparator for binary key comparison based on Arrow data type.

Tests

• BTreeFileFooterTest
• BTreeFileMetaSelectorTest
• BTreeIndexMetaTest
• KeySerializerTest

[leaves12138]

Thanks for migrating the BTree global index core components. I reviewed PR #74 at head 5130412cd9e90563f79f7836060b6f817a845636.

Treating the remaining prerequisite dependency state as expected migration-period drift, I did not find a blocking correctness issue in the migrated BTree definitions, file footer, index metadata, file meta selector, key serializer, and related tests. The migrated files match the existing C++ source snapshot aside from Apache license header updates.

Please make sure the dependent migration PRs are merged or rebased in, and run the relevant C++ build/tests from a clean checkout before final merge.