Let people choose their CI system when they create a solution #84

Today every generated solution gets a GitHub Actions ci.yml. Some people will want GitLab, Azure DevOps or something else as their default. GitHub stays the default until this is picked up.

Phase 1: Decide the shape

• Options: a --ci choice (github, gitlab, azure-devops, none) or one --include-<system> toggle per system, following the template feature flag rules
• Decide dependency updates for each system (Dependabot, Renovate or nothing) at the same time

Phase 2: Build it

• The same restore, format, build, test and 95% gate for each system
• Verify Template generates every choice
• Update ci-for-your-solution and each Set up CI page

Checks: the build pipeline passes, and each generated pipeline runs green on its system.