Added GetSize() const method for FixedStringType

By using ColumnLowCardinalityT<ColumnFixedString>, you lose the ability to conveniently know the fixed size parameter of ColumnFixedString. Adding this method fixes this issue.

clickhouse/types/types.h

@@ -251,6 +251,8 @@ class FixedStringType : public Type {
 
     std::string GetName() const { return std::string("FixedString(") + std::to_string(size_) + ")"; }
 
+    inline size_t GetSize() const { return size_; }
+
 private:
     size_t size_;
 };