Adds site-config configuration for RFC 969 intent detection, making the Intent Detection API endpoint and token configurable without code changes. Additionally, adds an option to hit multiple intent detection backends with the same query.

Previously, URL was hardcoded in code, so if the backend has changed, we had to redeploy sourcegraph.com.
As we iterate on intent detection, we want to be able to test multiple models in parallel, so this PR adds a setting for extra backends - if provided, additional .com -> backend requests will be sent, but the client-initiated request will not wait for those requests.

Closes AI-128.

Test plan

• tested locally - add

 "cody.serverSideContext": {
      "intentDetectionAPI": {
        "default": {
          "url": "http://35.188.42.13:8000/predict/linearv2"
        },
        "extra": [
          {
            "url": "http://35.188.42.13:8000/predict/linearv2"
          }
        ]
      }
    }

to experimentalFeatures in dev-private.