feat: add Dependency Firewall ecosystems (composer, conan, golang, nuget, cargo, swift, pub)

Adds ecosystem constants for the Dependency Firewall evaluate endpoint:

• composer (PHP)
• conan (C/C++)
• golang (Go)
• nuget (.NET)
• cargo (Rust)
• swift (Swift)
• pub (Dart)

Wire values match GitLab Package Registry naming conventions.

The Dependency Firewall REST API is being updated in parallel to accept these ecosystems, so the public docs at https://docs.gitlab.com/api/dependency_firewall/ do not yet list them.

• Accept eight additional ecosystems in the depen... (gitlab-org/gitlab!252580 - merged) • Michael Eddington • 19.4

A table-driven test covers all supported ecosystem constants, verifying each one serializes to the expected wire string on the request body.