Allow additional minute transfer for Users

What does this MR do?

When transferring the recently added Ci::Minutes::AdditionalPacks between namespaces, the service did not originally cater for namespaces which are a kind of User.

This meant the service would raise an error because user namespaces do not implement owner_ids, which is used to validate ownership between the two given namespaces.

This MR fixes the issue by using the appropriate method depending on the type of namespaces being used.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Ensure you have a namespace with some additional minutes:

  group = Group.last
  Ci::Minutes::AdditionalPack.create(namespace: group, number_of_minutes: 1000, purchase_xid: 'testing', expires_at: Date.current + 1.year)

1. Ensure one of your users is an owner of the group

  user = User.last
  group.add_owner(user)

1. Attempt to transfer additional minutes between the user and the group

  admin = User.find_by_username("root")
  Ci::Minutes::AdditionalPacks::ChangeNamespaceService.new(admin, group, user.namespace).execute

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?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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