By default, only YAML-defined bridge variables are passed to downstream
pipelines. With the forward keyword, it is now available to pass
manual pipeline variables to downstream pipelines.

- forward:yaml_variables is an existing behavior, by default it's true.
When true, YAML-defined bridge variables are passed to
downstream pipelines.
- forward:pipeline_variables is a new feature, by default it's false.
When true, manual pipeline variables are passed to downstream pipelines.

This is behind a feature flag ci_trigger_forward_variables.