Retrieve security dashboard URL used on Project Severity status report from backend - BE implementation

The following discussion from !35031 (merged) should be addressed:

• @afontaine started a discussion: (+2 comments)

  Hmm I've never really seen us try to construct a URL to something in Vue. Is there anyway we can get this path from the backend? If not, can we pull it out into a function and save some of it as constants instead?

Extra context

We have to construct the URL in the frontend since GraphQL misses this information. I believe the correct way to handle this should be to provide this information in GraphQL - for that reason I believe this task requires backend work.

Additional context from MR !35031 (comment 371695844)

A proper fix to this would be to actually include full dashboard URL within each project object under severityGroup.projects, but I think that will require some backend help.

Implementation plan

1. Extend Types::ProjectType (app/graphql/types/project_type.rb) to have security_dashboard_url property.
2. Extend specs to account for the URL field