Add variants_mode: local support to get a variants file for package

[traversaro]

Supersedes #114 . This implements O2 in #114 (comment) .

This PR adds a variants_mode key to vinca.yaml, that controls how rattler-build receives the
repository-level conda_build_config.yaml:

• global (default) leaves generated recipes unchanged. Builds must pass the
  root configuration explicitly, for example with
  rattler-build build ... -m ./conda_build_config.yaml.
• local generates a variants.yaml beside every recipe.yaml. Vinca converts
  legacy comment selectors to v1 selectors and keeps only pins used by that
  package. Builds must not pass the root configuration with -m, because an
  explicit variant file takes precedence over the recipe-local file.

In case variants_mode: local is used, it is possible to specify a per-package variant_overrides in pkg_additional_info.yaml:

my_ros_package:
  variant_overrides:
    c_stdlib_version:
      - '2.28'

this permits to build only a subset of a packages with an higher c_stdlib_version, as required for example in RoboStack/ros-jazzy#167 .

Note that switching from variants_mode: global to variants_mode: local requires changing some files in the ros-<distro> robostack repository, but does not strictly requires a full rebuild.