Use default config_id for completions endpoint

[rgstephens]

It would be nice if the completions endpoint was more compatible with the OpenAI endpoint so that tools that use this endpoint would work with guardrails without modification. I'd like to use the deep-chat widget for example.

My proposal is that if neither config_ids nor config_id are provided on the completions call, the value would be set to ["config"] as the default.

[drazvan]

Indeed. We could even use "model" as the configuration id. Thanks for the contribution!

[rgstephens]

For my use case, the "model" value need to be empty.