[ML] Exposing OpenAI URL field in services API

[jonathan-buttner]

This PR adds the url field to the services API for the openai service.

Fixes #126350

Testing

GET _inference/_services

Response

    {
        "service": "openai",
        "name": "OpenAI",
        "task_types": [
            "text_embedding",
            "completion",
            "chat_completion"
        ],
        "configurations": {
            "api_key": {
                "description": "The OpenAI API authentication key. For more details about generating OpenAI API keys, refer to the https://platform.openai.com/account/api-keys.",
                "label": "API Key",
                "required": true,
                "sensitive": true,
                "updatable": true,
                "type": "str",
                "supported_task_types": [
                    "text_embedding",
                    "completion",
                    "chat_completion"
                ]
            },
            "organization_id": {
                "description": "The unique identifier of your organization.",
                "label": "Organization ID",
                "required": false,
                "sensitive": false,
                "updatable": false,
                "type": "str",
                "supported_task_types": [
                    "text_embedding",
                    "completion",
                    "chat_completion"
                ]
            },
            "rate_limit.requests_per_minute": {
                "description": "Default number of requests allowed per minute. For text_embedding is 3000. For completion is 500.",
                "label": "Rate Limit",
                "required": false,
                "sensitive": false,
                "updatable": false,
                "type": "int",
                "supported_task_types": [
                    "text_embedding",
                    "completion",
                    "chat_completion"
                ]
            },
            "model_id": {
                "description": "The name of the model to use for the inference task.",
                "label": "Model ID",
                "required": true,
                "sensitive": false,
                "updatable": false,
                "type": "str",
                "supported_task_types": [
                    "text_embedding",
                    "completion",
                    "chat_completion"
                ]
            },
            "url": {
                "description": "The absolute URL of the external service to send requests to.",
                "label": "URL",
                "required": false,
                "sensitive": false,
                "updatable": false,
                "type": "str",
                "supported_task_types": [
                    "text_embedding",
                    "completion",
                    "chat_completion"
                ]
            },
            "dimensions": {
                "description": "The number of dimensions the resulting embeddings should have. For more information refer to https://platform.openai.com/docs/api-reference/embeddings/create#embeddings-create-dimensions.",
                "label": "Dimensions",
                "required": false,
                "sensitive": false,
                "updatable": false,
                "type": "int",
                "supported_task_types": [
                    "text_embedding"
                ]
            }
        }
    },

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[davidkyle]

LGTM

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x