Add new Rails/StrictLoadingAssociations cop

[Drenmi]

Rails 6.1 added a set of association strict loading tools to prevent N+1 queries. One of these is the ability to declare a has_many association to only strict load (failing to do so will raise an exception.) Reference.

Example:

class Author < ApplicationRecord
  has_many :posts, strict_loading: true
end

This PR adds a cop that enforces declaring a strict_loading option on has_many associations when using Rails 6.1 or newer. It is not concerned with what the value is, as setting this to true across the board is likely not good either.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[Drenmi]

I was a bit surprised that $hash ? produces an array with 0 or 1 elements, instead of nil or match. 🙂

[koic]

AFAIK, strict_loading has the following approaches in addition to defining in association.

• Specify in the application config: config.active_record.strict_loading_by_default = true
• Specify in model units: self.strict_loading_by_default = true
• Use as a method: Model.strict_loading.first

I'm not sure it's better approach to always force strict_loading to associations without considering these use cases.

[koic]

@Drenmi Can you handle the above? I'm concerned that the lack of the above points could possibly cause user confusion.