use buildpg to prepare function

[vincentsarago]

ref #68

@bitner let me know if that's what you envisioned.

~We now have another problem, we need to cast additional argument because at the application level we will get string. I'm not sure what's the best way to do this:~

• ~parse SQL code in python (seems possible using sqlparse)~
• ~use run a SQL code to get argument+type 🤷‍♂️~

[vincentsarago]

Last commit changed the way we handle function layers. This is now similar to https://github.com/urbica/martin#function-sources, where the application doesn't know the SQL function parameters but forward additional query parameter as a JSON param. This forward the responsibility of knowing how to parse the query to the user.

IMO this is the best option we have now. As mentioned before, we don't want to register the function to the database nor we want to parse the SQL code using python.

To Do

• [x] review from @bitner
• [x] update documentation