dialect/sql/schema: add field collation support (#1548)

* specific field collation support

* go generate ./...

dialect/sql/schema/schema.go

@@ -176,6 +176,7 @@ type Column struct {
 	Nullable   bool              // null or not null attribute.
 	Default    interface{}       // default value.
 	Enums      []string          // enum values.
+	Collation  string            // collation type (utf8mb4_unicode_ci, utf8mb4_general_ci)
 	typ        string            // row column type (used for Rows.Scan).
 	indexes    Indexes           // linked indexes.
 	foreign    *ForeignKey       // linked foreign-key.
@@ -307,6 +308,13 @@ func (c Column) supportDefault() bool {
 	}
 }
 
+// collation adds `COLLATE collation_name`.
+func (c *Column) collation(b *sql.ColumnBuilder) {
+	if c.Collation != "" {
+		b.Attr("COLLATE " + c.Collation)
+	}
+}
+
 // unique adds the `UNIQUE` attribute if the column is a unique type.
 // it is exist in a different function to share the common declaration
 // between the two dialects.