[wip] add cypher frontend

[phall1]

pgGraph today exposes graph queries as PostgreSQL SQL functions:

SELECT * FROM graph.traverse(
    'public.people'::regclass, 'alice',
    /*max_depth=*/ 2, /*edge_types=*/ ARRAY['KNOWS'],
    /*direction=*/ 'outgoing', ...
);

For complex queries (ex multi hop patterns with predicates), cypher (& GQL) more expressive.

Adding a new function & support for graph.cypher(text, jsonb) that:

• accepts an openCypher v9 query string
• parses it through the cyrs frontend, and
• dispatches each plan operator to either pgGraph's existing in-memory engine (reads) or to SPI-issued DML against the registered source tables (writes).

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This is a draft for visibilty (the markdowns will be stripped when pr is ready).

cyrs is still in development, but has a pretty robust test suite. Also open to contributions over there