Add Support for Mixin Composition in Colcon Mixin

I am C Innocent Pious, a final-year B.Tech Computer Science student from Chennai with hands-on experience in the colcon mixin codebase. Prior to this proposal, I identified and fixed a multi-year bug in multi-mixin argument merging (Issue #40, PR #59), adding six regression tests with all tests passing. This project introduces mixin composition to colcon mixin, allowing a mixin to declare dependencies on other mixins via a "mixin" key. These dependencies will be resolved automatically using a dependency graph, topological ordering, and cycle detection, eliminating the need for users to manually specify transitive dependencies in every colcon invocation. The deliverables include a dependency resolution system, cycle-safe ordering mechanism, well-defined argument merge semantics, comprehensive test coverage, and updated documentation integrated into the upstream repository.