Add support for Stored Procedures

[laurenceisla]

Resolves part of #1210. As noted in the issue, nested transactions won't be implemented

• [x] Identify stored procedures in the schema cache
• [ ] Query the data from the procedure (INOUT and OUT), use https://github.com/steve-chavez/postgrest/commit/8ac5a11b35a98685e288ced786f6377de8f90641 as reference
• [ ] Add tests
• [ ] Optimize code
• [ ] Modify changelog

[steve-chavez]

We can fully support stored procedures transactions with the idea mentioned here.

[steve-chavez]

On 9.0.0, when trying to call a procedure, we used to fail with:

{
    "hint": "To call a procedure, use CALL.",
    "details": null,
    "code": "42809",
    "message": "api.do_nothing() is a procedure"
}

Now we fail with a schema cache error:

Could not find the api.do_nothing() function in the schema cache

I think both errors are imprecise, ideally we should throw a 501 Not Implemented and mention procedures are not supported yet. Also maybe suggest moving to a function since we already wrap it with a transaction.