Fixes 5026 Add PostgreSql grammar for GIN

[griffio]

fixes #5026

Basic GIN support for json columns

CREATE INDEX gin_alpha_1 ON json_gin USING GIN (alpha);

support gin operator classes (includes non json operators for completeness https://www.postgresql.org/docs/current/gin-builtin-opclasses.html)

CREATE INDEX gin_alpha_1 ON json_gin USING GIN (alpha jsonb_ops);

An operator class can be specified for each column of an index

CREATE INDEX gin_alpha_beta_5 ON json_gin USING GIN (alpha jsonb_path_ops, beta jsonb_ops);

Added fixture tests to check basic syntax permutations