`moose function init` behaviors

[callicles]

1. If a streaming function already exists with the same source and destination data models specified in the command:
   1. The CLI should raise a warning: Warning: Streaming Function -> already exists.
2. If a data model specified in the command does not exist:
   1. it creates the appropriate folders and files,
   2. but does not create the data model in the /datamodels folder