Namespace substitution variables

[disrupted]

The global configuration variables can be namespaced under the config key, such as ${config.kafka_brokers}, ${config.schema_registry.url}. Same thing could be considered for pipeline variables, e.g. ${pipeline_name} → ${pipeline.name}.
This would make it more uniform with the existing ${component.<key>} variables.

Furthermore, we might consider deprecating the hardcoded aliases ${output_topic_name} and ${error_topic_name} afterwards. (#383)