Make GraphQL enums a little bit nicer

What does this MR do?

Just a tiny improvement so our GraphQL enum usage can look a bit tidier:

Adds an enum method to GraphQL enums that works pretty much the same as ActiveRecord enum's pluralized value holder method.

Example: Given a GraphQL enum class defined as such:

module Types
  class AnEnum < BaseEnum
    graphql_name 'An'
    description 'AnEnum'

    value 'TEST', 'A test?', value: 3
    value 'other'
    value 'NORMAL'
  end
end

[9] pry(main)> Types::AnEnum.enum
=> {"test"=>3, "other"=>"other", "normal"=>"NORMAL"}
[10] pry(main)> Types::AnEnum.enum[:test]
=> 3
[11] pry(main)> Types::AnEnum.enum[:normal]
=> "NORMAL"

Screenshots

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team