PARQUET-758: Add Float16/Half-float logical type

Type involves a trade-off of reduced precision,
in exchange for more efficient storage.

LogicalTypes.md

@@ -245,6 +245,14 @@ comparison.
 To support compatibility with older readers, implementations of parquet-format should
 write `DecimalType` precision and scale into the corresponding SchemaElement field in metadata.
 
+### FLOAT16
+
+The `FLOAT16` annotation represents half-precision floating-point numbers in the 2-byte IEEE little-endian format.
+
+Used in contexts where precision is traded off for smaller footprint and potentially better performance.
+
+The primitive type is a 2-byte fixed length binary.
+
 ## Temporal Types
 
 ### DATE

src/main/thrift/parquet.thrift

@@ -232,6 +232,7 @@ struct MapType {}     // see LogicalTypes.md
 struct ListType {}    // see LogicalTypes.md
 struct EnumType {}    // allowed for BINARY, must be encoded with UTF-8
 struct DateType {}    // allowed for INT32
+struct Float16Type {} // allowed for FIXED[2], must encoded raw FLOAT16 bytes
 
 /**
  * Logical type to annotate a column that is always null.
@@ -342,6 +343,7 @@ union LogicalType {
   12: JsonType JSON           // use ConvertedType JSON
   13: BsonType BSON           // use ConvertedType BSON
   14: UUIDType UUID           // no compatible ConvertedType
+  15: Float16Type FLOAT16     // no compatible ConvertedType
 }
 
 /**