WIP: Support pattern matching in ServiceResponse

What does this MR do?

This MR adds pattern matching support for ServiceResponses. There are two aspects to this: subclasses to support dispatching on class, and deconstruct_keys to support binding instance fields.

The concrete case is to be able to do the following:

case service.execute
in ServiceResponse::Success(payload: { thing: x })
  do_something_with(x)
in ServiceResponse::Error(message: msg)
  oh_no!(msg)
end

i.e. pattern matching is much more useful when using it to branch and bind at the same time.

Without the subclasses, but making use of deconstruct_keys, the following syntax is also available:

case service.execute
in { state: :success, payload: { thing: x } }
  do_something_with(x)
in { state: :error, message: msg }
  oh_no!(msg)
end

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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