feat(checkpoint-postgres): Add support for providing a custom schema during initialization

[nick-w-nick]

This PR adds support for specifying a custom schema in the checkpoint-postgres module during initialization. Instead of relying on the default public schema used by Postgres or modifying the connection string’s search path (which can be unreliable at best), SQL queries now explicitly reference the provided schema, ensuring compatibility across all PostgreSQL versions and configurations, while still allowing the original module to stay backward compatible with previous versions since public was already the default schema.

// you can optionally pass a configuration object as the second parameter
const checkpointer = PostgresSaver.fromConnString("postgresql://...", {
  schema: "schema_name" // defaults to "public"
});

Key Changes

• Custom Schema Support
  • Added an extendable options parameter to specify a schema when creating a PostgresSaver instance via the fromConnString method
  • Consolidated SQL queries and migration scripts into reusable functions that accept a schema name
• Documentation & Testing
  • Improved JSDoc comments for fromConnString with usage example
  • Enhanced integration tests to validate behavior with both the default and a custom schema