alternator: add fallback serialization for all types

While most types (e.g. boolean) are not valid key types for alternator users,
system tables derived from Scylla may still use this type for keys,
e.g. system_auth.roles. Note that types which are not directly
supported by alternator (e.g. double) will not be representable
out-of-the-box - instead, they simply fall back to string, which is both
human-readable and supported by alternator.

alternator/serialization.cc

@@ -153,7 +153,9 @@ std::string type_to_string(data_type type) {
     };
     auto it = types.find(type);
     if (it == types.end()) {
-        throw std::runtime_error(format("Unknown type {}", type->name()));
+        // fall back to string, in order to be able to present
+        // internal Scylla types in a human-readable way
+        return "S";
     }
     return it->second;
 }
@@ -205,8 +207,11 @@ rjson::value json_key_column_value(bytes_view cell, const column_definition& col
         auto s = to_json_string(*decimal_type, bytes(cell));
         return rjson::from_string(s);
     } else {
-        // We shouldn't get here, we shouldn't see such key columns.
-        throw std::runtime_error(format("Unexpected key type: {}", column.type->name()));
+        // Support for arbitrary key types is useful for parsing values of virtual tables,
+        // which can involve any type supported by Scylla.
+        // In order to guarantee that the returned type is parsable by alternator clients,
+        // they are represented simply as strings.
+        return rjson::from_string(column.type->to_string(bytes(cell)));
     }
 }