Ban and unban user via API

What does this MR do?

Epic: &5741

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/327354

Malicious actors create many issues in public projects/groups for spam. We need a way to be able to hide their issues until they can be reviewed and deemed safe. So we've introduced the ability for admins to ban users, which blocks the user and hides their contributions. Hidden issues cannot be accessed by non-admins. Admin users can still see the hidden issues so they can review the issues' contents.

For integrations, we need a way to set a user to this state via the API. This MR introduces the ability for instance administrators to ban and unban a user via the API.

A user cannot be banned via the API if the user is LDAP blocked, deactivated, or already banned or blocked. A user cannot be unbanned via the API if they are LDAP blocked, deactivated, or not already banned.

To ban a user: curl --request POST --header "PRIVATE-TOKEN: $TOKEN" "http://127.0.0.1:3000/api/v4/users/<user id>/ban"

Unban a user: curl --request POST --header "PRIVATE-TOKEN: $TOKEN" "http://127.0.0.1:3000/api/v4/users/<user id>/unban"

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

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