Reintroduce `Runtime` class (with amendments)

What does this MR do?

Follow up from https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8729

This is a revert of !21882 (merged), which was a revert of !20294 (merged).

The original changes have been largely restored, however, with the following amendments:

• revert to runtime check logic that was applied prior to !20294 (merged) (but keep API intact)
• when multiple or unknown runtimes are detected in the initializer, track an error into Sentry
• rename app_server? to web_server? (this seems to be the more commonly adopted term)
• detect rspec as a runtime?
• detect rails runner as a runtime?

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

The original MR shipped with issues when running in an omnibus deployment.

How can we make sure this doesn't happen again?