Officially support labels for pipelines and agents to improve pipeline picking.
* add pipeline labels
* update, improve docs and add migration
* update proto file
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closes#304 & #860
Dropped support for `DRONE_*` environment variables in pipeline steps. Pipeline meta-data can be accessed with `CI_*` variables.
- `CI_*` prefix replaces `DRONE_*`
- `CI` value is now `woodpecker`
- `DRONE=true` has been removed