Generated new Rails Engine scaffolding using `rails plugin new --full`.

Provide gemspec and initial implementation of an `Integrations::Phorge`
model derived from GitLab CE's `Integrations::BaseIssueTracker` base
class. For now, the integration is simple. It defines a custom regex to
render T* task IDs as links to a configured Phorge site.

The bare minimal has been stubbed out in test/dummy/app/models to
support the basic test cases.

Bug: T337570