feat: Add support for OPTIONAL MATCH keyword

**Is your feature request related to a problem? Please describe.**
No

**Describe the solution you'd like**
Add suport for OPTIONAL MATCH in GQL which is equivalent to an outer join in SQL

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered. Although I can't find the exact documentation in the ISO standard which states that OPTIONAL MATCH is part of it, the grammar provided in [this](https://github.com/TuGraph-family/gql-grammar?tab=readme-ov-file) link provided an OPTIONAl MATCH statement.

**Additional context**
Other popular database like Neo4j also supports this statement: https://neo4j.com/docs/cypher-manual/current/clauses/optional-match/

Start by reading the linked TuGraph-family GQL grammar and Neo4j's OPTIONAL MATCH documentation to establish the intended syntax and semantics. Then locate GeaFlow's GQL parsing and query execution entry points; done means OPTIONAL MATCH is accepted and behaves like an outer join, with coverage for the supported cases.

Written by the indexing model from the issue text.