Support for UDFs as GraphQL queries/mutations

[imor]

What kind of change does this PR introduce?

New feature and docs update.

What is the current behavior?

User defined functions are not available through GraphQL.

What is the new behavior?

User defined functions are available as fields on the query and mutation root types.

Additional context

Tasks

Add support for base types:

• Add UDF metadata to sql context.
• Expose UDFs in introspection schema.
• Parse UDF fields on mutation and query objects.
• Transpile & execute UDF field requests.
• Add tests

Add support for composite types:

• Add UDF metadata to sql context.
• Expose UDFs in introspection schema.
• Parse UDF fields on mutation and query objects.
• Transpile & execute UDF field requests.
• Add tests

Misc:

• Remove tables->functions key and use the new functions in sql context for columns extended with functions.
• Handle function name clashes with other fields. E.g. if there is a table named account and a function named accountCollection.
• Accept args for function which return a single table row.
• Add support for argument name inflection.
• Update docs.

Resolves #222

[imor]

this looks really great. the asks would be:

• Add in the return type transforms so e.g. bigint gets returned as as string and json as an escaped json string
• Filter out the functions that aren't supported yet rather than leaving their behavior as undefined

+1 on your API design decisions

In case you hadn't seen these, good reading: https://www.graphile.org/postgraphile/functions/ https://www.graphile.org/postgraphile/custom-mutations/

Yeah, I have read these. Actually these pages influenced a lot of the design choices for our implementation.