Make it even easier to add support for custom GF fields

[justlevine]

Is your feature request related to a problem? Please describe.

People choose Gravity Forms for its extensibility. While v0.10 made much progress to help devs support custom form fields, it still requires several filters to make work.

We should explore ways to make this even easier in WPGraphQL for GF, ideally by offering a single function that handles all the heavy lifting.

Describe the solution you'd like

In theory, a register_graphql_gf_form_field() function that resembles its namesake.

The function should let users

1. Define the necessary GraphQL fields on the autogenerated FormField object.
2. Define the FormFieldValueInput field for mutation support.
3. Let you overwrite the autogenerated FormField object type name.

Function arguments and usage should be as similar to core WPGraphQL register_graphql_{} functions as possible.