Verify how to parse some special default values for arrays, timezones, etc.

[laurenceisla]

Need to verify how to parse some special default values, e.g. arrays have two types of defaults: {1,2,3}::integer[] and ARRAY[1,2,3] (which can be parsed) but composite types or even timestamps sometimes return complex defaults which are not accepted by the OpenAPI spec (e.g.: timezone('utc'::text, now())). Maybe add another x-pgrst-sql-default for complex types and leave default for integers, text, etc.

Originally posted by @laurenceisla in https://github.com/laurenceisla/postgrest-openapi/pull/3#discussion_r1198483329