Add ability hide GraphQL fields via feature flag

What does this MR do?

This adds the functionality of easily hiding a GraphQL field behind a feature flag.

This can be done like so:

    field :id, GraphQL::STRING_TYPE,
      null: false,
      description: 'ID of the item',
      feature_flag: :test_feature

When the feature_flag option is given, we:

• Append the description to indicate it's behind a feature flag.
• Check if the Feature flag is enabled, by overriding the visible? method. If it is not enabled, we return a Field 'xxx' doesn't exist on type 'xxx' error.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #197129 (closed)