feat(migration): add support for altering column nullability

* Implement ExecuteAlterColumnNullability function
* Create alter_column_nullability template
* Add test for altering column nullability behavior

pkg/writers/pgsql/templates.go

@@ -116,6 +116,14 @@ type AlterColumnDefaultData struct {
 	DefaultValue string
 }
 
+// AlterColumnNullabilityData contains data for alter column nullability template
+type AlterColumnNullabilityData struct {
+	SchemaName string
+	TableName  string
+	ColumnName string
+	NotNull    bool
+}
+
 // CreatePrimaryKeyData contains data for create primary key template
 type CreatePrimaryKeyData struct {
 	SchemaName     string
@@ -331,6 +339,16 @@ func (te *TemplateExecutor) ExecuteAlterColumnDefault(data AlterColumnDefaultDat
 	return buf.String(), nil
 }
 
+// ExecuteAlterColumnNullability executes the alter column nullability template
+func (te *TemplateExecutor) ExecuteAlterColumnNullability(data AlterColumnNullabilityData) (string, error) {
+	var buf bytes.Buffer
+	err := te.templates.ExecuteTemplate(&buf, "alter_column_nullability.tmpl", data)
+	if err != nil {
+		return "", fmt.Errorf("failed to execute alter_column_nullability template: %w", err)
+	}
+	return buf.String(), nil
+}
+
 // ExecuteCreatePrimaryKey executes the create primary key template
 func (te *TemplateExecutor) ExecuteCreatePrimaryKey(data CreatePrimaryKeyData) (string, error) {
 	var buf bytes.Buffer