fix(gatsby): Extend fields when merging types

[stefanprobst]

When merging GraphQL types, we are currently using graphql-compose, which internally uses addFields, which will replace the full fieldConfig on an existing field with a new one. This is a problem e.g. for gatsby-plugin-schema-snapshot when merging the field type, any existing resolver will be lost:

// gatsby-node.js
exports.createSchemaCustomization = ({ actions }) =>
  actions.createTypes(`
    type Mdx implements Node {
      # This will overwrite the full field config on Mdx.body with { type: String }
      # so the resolver defined by gatsby-plugin-mdx is lost
      body: String!
    }
  `)

This PR changes this to use extendField instead of addFields

Fixes: #18095

[freiksenet]

Glad to have you back!