Add directives to mutate multiple models at once

[spawnia]

Resolves https://github.com/nuwave/lighthouse/issues/2091

• [x] Added or updated tests
• [x] Documented user facing changes
• [x] Updated CHANGELOG.md

Changes

Adds the directives @createMany, @updateMany and @upsertMany. They correspond to @create, @update and @upsert, but operate on multiple models at once.

Breaking changes

No explitly public API changed, but MutationExecutorDirective splits into OneModelMutationDirective and ManyModelMutationDirective.