[Backend] Add custom endpoint for new registrations

What does this MR do?

This MR adds backend code required for Step 1 of Improved free trial sign-up: https://gitlab.com/gitlab-org/gitlab-ee/issues/13233.

It adds a new endpoint for Trial registrations. The API inherits from the existing sign up at RegistrationsController - by including only additional logic on top of it.

Additional logic:

1. Additionally includes 3 parameters to accepted params: first_name, last_name, skip_confirmation
2. The trial sign up form accepts first and last name from user. Since name is a mandatory field in users table, it would be derived from first_name and last_name.

Since trials is application only for GL.com, this endpoint is defined under EE.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

REF: #13357 (closed)

REF: #13233 (closed)

CE changes: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32233