Cloud License: Subscription activation service

What does this MR do?

Adds a service which would call CustomerDot's activate API. If successful, the service would persist the returned authentication_token in ApplicationSetting for future syncs. The token should be encrypted.

This is currently unused as it is the first step of the Cloud License epic, an experimental feature for self managed instance admins. This is guarded by application setting, which is the same as feature flag, therefore I didn't add a changelog here as it is not user facing yet.

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.
• [-] 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

Related to #273501