fix(gatsby): Add __typename and id fields in queries to match previous behavior of relay-compiler

[vladar]

Description

We introduced an unintentional BC break in #19665 (when replaced Relay compiler with our own solution)

Before that PR relay compiler was adding extra fields to queries automatically:

1. An id field if it existed on the type
2. A __typename field if the type was a union or an interface.

In other words the following query:

{
  unionField {
    ... on TypeWithId {
      foo
    }
  }
}

was transformed to:

{
  unionField {
    __typename
    ... on TypeWithId {
      id
      foo
    }
  } 
}

Current PR mimics Relay compiler behavior and restores backwards compatibility.

Documentation

This "feature" is not documented and shouldn't be. It is not recommended to rely on it as it will be removed in Gatsby v3.0

Related Issues

Fixes #20290

[freiksenet]

Begrudgingly accepted :D

[vladar]

Published in [email protected]

[afc163]

#20943 (comment)