Add rake task `rubocop:todo:generate`

What does this MR do?

This MR abstracts bundle exec rubocop --auto-gen-config --auto-gen-only-exclude --exclude-limit=100000 in a rake task rubocop:todo:generate.

Contributes to #294273 (closed).

This MR is a step towards potentially splitting up .rubocop_todo.yml into multiple files (one per cop) and removing the ever-growing rubocop_manual_todo.yml.

How to setup and validate locally (strongly suggested)

bundle exec rake -T rubocop
rake rubocop                # Run RuboCop
rake rubocop:auto_correct   # Auto-correct RuboCop offenses
rake rubocop:todo:generate  # Generate RuboCop todos
rake spec:rubocop           # Run the code examples in spec/rubocop

bundle exec rake rubocop:todo:generate

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• [-] This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] 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