Add current user to comparison services for permissions

What does this MR do?

Issue ticket https://gitlab.com/gitlab-org/gitlab-ee/issues/14780

https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/942845/?query=is:unresolved

We would like to add current_user to the comparison services objects so that when we serialise we can check their permissions. Hopefully this will solve the issue that we see in staging. More details are in the ticket.

CompareDependencyScanningReportsService < ::Ci::CompareReportsBaseService < ::BaseService
CompareContainerScanningReportsService < ::Ci::CompareReportsBaseService < ::BaseService
CompareSastReportsService < ::Ci::CompareReportsBaseService < ::BaseService

CompareMetricsReportsService doesn't need current_user at the moment

CompareMetricsReportsService  < ::Ci::CompareReportsBaseService < ::BaseService

::BaseService has already attribute called current_user we are just setting this attribute in this MR

Does this MR meet the acceptance criteria?

• http://<local_instance>/gitlab-org/ruby-bundler/merge_requests/<mr_number>/container_scanning_reports.json
• http://<local_instance>/gitlab-org/ruby-bundler/merge_requests/<mr_number>/dependency_scanning_reports.json
• http://<local_instance>/gitlab-org/ruby-bundler/merge_requests/<mr_number>/sast_reports.json

Should work

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