Store an inference-provider credential

Store (or overwrite) the miner's credential for the given provider. Validates the credential before persisting so bad input fails here rather than at first claim_work. The first stored provider becomes the miner's default; switching default afterward goes through the dedicated PATCH endpoint.

curl -X POST "https://loading/v1/miner/inference-auth/string" \  -H "Content-Type: application/json" \  -d '{    "credential": "string"  }'

{
  "property1": true,
  "property2": true
}

{
  "detail": [
    {
      "loc": [
        "string"
      ],
      "msg": "string",
      "type": "string",
      "input": null,
      "ctx": {}
    }
  ]
}