Add subgroups and user namespaces spec examples for maven

🍰 Context

The package registries can be available on 3 different "levels":

• Project level (accessed with a project id)
• Group level (accessed with a group id)
• Instance level (accessed with a fixed url)

Each level will pull packages within the target. For example, accessing a group level registry will make all packages (including those in subgroups / subprojects) available.

In #322150, it has been suggested to properly have tests for

• Group level
  • We want to make sure that packages in subgroups are available.
• Instance level
  • We also want to make sure that packages in subgroups are available.
  • We want to make sure that packages in a user namespace are available.

On top of that, we need to play around with visibilities. A public package will be available to all users. Obviously, that is not the case for private packages.

This effort to add more tests has been broken down in package types. This MR adds the relevant spec examples to the Maven API.

🔬 What does this MR do?

• Add examples to the maven API for
  • packages in public, private, internal subgroups (for the group and instance level endpoint)
  • packages in public, private user namespaces (for the instance level endpoint)

🎥 Screenshots (strongly suggested)

n / a

🛃 Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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