fix(gatsby): print childOf directive for implicit child fields

[vladar]

Description

Fixes a bug with schema printing of inferred childOf directive. Before this PR if we print a schema with infered child fields (e.g. File.childImageSharp) it won't include childOf directive:

type ImageSharp implements Node @dontInfer {
  id: ID!
}

After this PR it correctly prints this:

type ImageSharp implements Node @dontInfer @childOf(types: ["File"]) {
  id: ID!
}

This fixed variant will correctly wotk in Gatsby v3. The variant before this PR will fail to work in Gatsby v3 (the printed schema snapshot won't add the File.childImageSharp field).

Potentially a breaking change :/

Worked around breaking change via #28656

See details (no longer actual)

Requires a bit of investigation. To illustrate a possible break. With this change, we automatically apply the `childOf` extension (instead of inferring child fields):

type Foo implements Node @childOf(types: ["Bar", "Baz"], many: true) {
  id: ID!
}

The many argument here is what is problematic. Before this change (with simple inference) we could get these two types (at least theoretically):

type Bar {
  childFoo: Foo
}
type Baz {
  childrenFoo: [Foo]
}

so in one case - singular Bar.childFoo in the other case - array Baz.childrenFoo. After this change, we will always have both singular or both arrays. Mixing is not allowed with this directive.

The many argument applies to all parent types. Not sure if it is intentional or an oversight.

Ways to fix this

1. Suggested by @pieh : Always add both child{Field} and children{Field} to the schema and deprecate many option altogether. Wih this change child{Field} will always return the first child (as listed in node children array)

2. Support the new syntax in childOf directive where we can define many per type. Something like:

type Foo implements Node @childOf(parents: [{name: "Bar", many: false}, { name: "Baz", many: true }]) {
  id: ID!
}

Related issues

Fixes #19674

[vladar]

The essence of this PR: instead of adding fields directly we add the childOf directive here. And fields are always added in a single place now - where this directive is processed (in addConvenienceChildrenFields):

gatsby/packages/gatsby/src/schema/schema.js

Lines 972 to 975 in 76a3b57

	children.forEach(child => {
	typeComposer.addFields(createChildrenField(child.getTypeName()))
	typeComposer.addFields(createChildField(child.getTypeName()))
	})

[pieh]

Looks great! That was quite a journey to get to this point (including other PR(s) on the way heh and possibly some ahead (weird behaviour of update option on schema snapshot plugin ;) ).

Let's get this in!