feat(gql): Trusted Documents support for CedarJS Mailer

[ladderschool]

Studio is broken for the mail feature if trusted documents store is enabled. See redwoodjs/studio#75

This fixes the issue:

This was the first initialization of studio with a test project, as you can see the second example is not showing and there's a copy remnant there. With trusted documents it's not synchronizing effectively.

This shows that studio synchronizes effectively after the changes in this PR.

EDIT by Tobbe:
I wanted to surface this really informative code comment:

/**
 * When using Studio, we want to allow the `resyncMailRenderers` and
 * `resyncMailTemplate` mutations to be executed without a persisted operation.
 * This is only allowed in local development, ensure you have
 * NODE_ENV=development set.
 *
 * This is because the `resyncMailRenderers` mutation is a special case that is
 * used by Studio to sync mail renderers from the mailer configuration.
 *
 * This function checks if the request is for the `resyncMailRenderers` or
 * `resyncMailTemplate` mutations and has the correct headers.
 *
 * If you need Studio in production, you just have to add the above mutations to
 * your SDL schema and as dummy resolvers so Trusted Documents can pick them up.
 */

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