✨ Add thresholding for detector results

[evaline-ju]

Following #51 , score thresholding for detector results will be done by the orchestrator. Each detector will need a configured default_threshold, expected to be added when the detector is added to the orchestrator config.

If a user provides a threshold through the API, this will be used. Otherwise the default threshold will be used.

Example config parts API that should give the same output if default_threshold: 0.5 were configured for detector model_name

"guardrail_config": {
        "input": { "models": {}},
        "output": {"models":{"model_name": {}}}
    },

"guardrail_config": {
        "input": { "models": {}},
        "output": {"models":{"model_name": {"threshold": 0.5}}}
    },

This PR also adds documentation/comments for the config structs and updates the validation error for request validation from #42.

For: #24

[declark1]

Looks good, just once last suggested change inline.

[declark1]

LGTM!