Add map data type support for Cassandra (#67)

Motivation:

The Cassandra client previously supported scalar types and arrays, but
lacked support for map types which are a fundamental Cassandra
collection type. Maps are commonly used for storing key-value pairs in
Cassandra schemas (e.g., user preferences, metadata, configuration
settings). Without map support, users had to work around this limitation
or couldn't use certain Cassandra features effectively.

Modifications:

- Created Statement+Maps.swift with map binding logic for all 63 type
combinations (7 key types × 9 value types)
- Added all 63 map enum cases to Statement.Value enum
- Implemented bindMap<K,V> helper using CASS_COLLECTION_TYPE_MAP
- Created Data+Maps.swift with map reading logic for all 63 combinations
- Implemented toMap<K,V> using cass_iterator_from_map(),
cass_iterator_get_map_key(), and cass_iterator_get_map_value()
- Added 63 map properties on Column and 126 convenience methods on Row
- Extended testMapTypes to comprehensively test all 63 map combinations
- Organized map-related code in separate extension files to keep
codebase clean

Supported key types: Int8, Int16, Int32, Int64, String, UUID,
TimeBasedUUID
Supported value types: Int8, Int16, Int32, Int64, Float32, Double, Bool,
String, UUID

Result:

Users can now bind and read all Cassandra map types with full type
safety. The implementation follows the same pattern as arrays, providing
consistent API ergonomics. All 63 map type combinations are tested and
verified to work correctly with round-trip insert/read operations.

Author: ericraio