result: add "Custom" metadata type

CQL protocol allows sending custom types identified by their
string representation. One of the cases in which the trick is used
is when Cassandra returns duration columns to a CQLv4 client, which
is not supposed to know this type.

Refs #364

Closes #368

Author: psarna

scylla/src/frame/response/result.rs

@@ -41,6 +41,7 @@ pub struct TableSpec {
 
 #[derive(Debug, Clone)]
 pub enum ColumnType {
+    Custom(String),
     Ascii,
     Boolean,
     Blob,
@@ -376,6 +377,10 @@ fn deser_type(buf: &mut &[u8]) -> StdResult<ColumnType, ParseError> {
     use ColumnType::*;
     let id = types::read_short(buf)?;
     Ok(match id {
+        0x0000 => {
+            let type_str: String = types::read_string(buf)?.to_string();
+            Custom(type_str)
+        }
         0x0001 => Ascii,
         0x0002 => BigInt,
         0x0003 => Blob,
@@ -534,6 +539,12 @@ fn deser_cql_value(typ: &ColumnType, buf: &mut &[u8]) -> StdResult<CqlValue, Par
     }
 
     Ok(match typ {
+        Custom(type_str) => {
+            return Err(ParseError::BadData(format!(
+                "Support for custom types is not yet implemented: {}",
+                type_str
+            )));
+        }
         Ascii => {
             if !buf.is_ascii() {
                 return Err(ParseError::BadData("String is not ascii!".to_string()));