[Bug] `elasticgraph-apollo` causes confusing `rake` warning

[myronmarston]

Describe the bug

When using elasticgraph-apollo in a project, the following warning is issued:

$ bundle exec rake
WARNING: 3 GraphQL resolver(s) have been registered but are unused:
  - apollo_entity_ref
  - apollo_entity_ref_list
  - apollo_entity_ref_paginated

The warning goes away if the new APIs added to EG v0.19.3.0 are used, but they are optional and rarely used. It's confusing because the end user doesn't know what those resolvers are and doesn't know how to address the warning.

To Reproduce:

1. Bootstrap a new EG project
2. Add elasticgraph-apollo
3. Run bundle exec rake

Expected behavior:

No warnings.