Recommend adding support for PGQL

ref: [https://pgql-lang.org/spec/2.1/](url)

The reasons are as follows:
1. It is a standardized language, which facilitates learning and promotion;
2. PGQL naturally integrates well with SQL;
3. It contributes to better integration with relational databases such as Oracle/Spanner.

No implementation file or test is named. Start with the PGQL 2.1 specification and inspect GeaFlow's existing query-language entry points and database integration; done requires an agreed support scope plus tests covering the selected PGQL behavior.

Written by the indexing model from the issue text.