Feature request: add/remove UDF through API/CLI without restarting Kapacitor.

As more and more alert requirements come to us, there is need to add/remove UDF to support these requirements frequently.

However, by modifying the UDF nodes in configuration file and restarting the Kapacitor, the existing alerts will be affected. We would like to have a more comfortable way to do this.

Thanks:)

Start by tracing the API and CLI entry points for UDF configuration and the configuration-file handling for UDF nodes. Define how add and remove operations should work at runtime, then verify that existing alerts remain unaffected without restarting Kapacitor.

Written by the indexing model from the issue text.