feat(relations): 🎉 add flatten schema option for output

* Introduce `--flatten-schema` flag to convert, merge, and split commands.
* Modify database writing functions to support flattened schema names.
* Update template functions to handle schema.table naming convention.
* Enhance PostgreSQL writer to utilize flattened schema in generated SQL.
* Update tests to ensure compatibility with new flattening feature.
* Dependencies updated for improved functionality.

pkg/writers/pgsql/templates/create_foreign_key.tmpl

@@ -1,10 +1,10 @@
-ALTER TABLE {{quote_ident .SchemaName}}.{{quote_ident .TableName}}
+ALTER TABLE {{qual_table .SchemaName .TableName}}
   DROP CONSTRAINT IF EXISTS {{quote_ident .ConstraintName}};
 
-ALTER TABLE {{quote_ident .SchemaName}}.{{quote_ident .TableName}}
+ALTER TABLE {{qual_table .SchemaName .TableName}}
   ADD CONSTRAINT {{quote_ident .ConstraintName}}
   FOREIGN KEY ({{.SourceColumns}})
-  REFERENCES {{quote_ident .TargetSchema}}.{{quote_ident .TargetTable}} ({{.TargetColumns}})
+  REFERENCES {{qual_table .TargetSchema .TargetTable}} ({{.TargetColumns}})
   ON DELETE {{.OnDelete}}
   ON UPDATE {{.OnUpdate}}
   DEFERRABLE;