datasets api

## Summary
- Clean up datasets API to work with uri & files.

- Example client usage
```python
client.datasets.create(
    uri="file://mydata.jsonl",
    schema="jsonl_messages",
    # optional
    dataset_id="my_first_train_data"
)

# ecosystem enablement ( get huggingface dataset easily )
client.datasets.create(
    uri="huggingface://tatsu-lab/alpaca",
    uri_params={"split": "train"},
    schema="jsonl_messages",
    metadata={"label": "v/finetuning/test"},
)

# [ in the future with stronger story for authentication ]
client.datasets.create(
    uri="s3://bucket/mydata.jsonl", <-- signed s3 url
    uri_parms={...},
    schema="jsonl_messages",
)
```


## Test Plan:
- Only API spec updates, implementation will be updated in follow PRs.

llama_stack/apis/datasets/datasets.py

@@ -4,6 +4,7 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+from enum import Enum
 from typing import Any, Dict, List, Literal, Optional, Protocol
 
 from pydantic import BaseModel, Field
@@ -14,9 +15,26 @@ from llama_stack.apis.resource import Resource, ResourceType
 from llama_stack.schema_utils import json_schema_type, webmethod
 
 
+@json_schema_type
+class Schema(Enum):
+    """
+    Schema of the dataset. Each type has a different column format.
+    :cvar jsonl_messages: The dataset is a JSONL file with messages. Examples:
+        {
+            "messages": [
+                {"role": "user", "content": "Hello, world!"},
+                {"role": "assistant", "content": "Hello, world!"},
+            ]
+        }
+    """
+
+    jsonl_messages = "jsonl_messages"
+    # TODO: add more schemas here
+
+
 class CommonDatasetFields(BaseModel):
-    dataset_schema: Dict[str, ParamType]
+    schema: Schema
-    url: URL
+    uri: str
     metadata: Dict[str, Any] = Field(
         default_factory=dict,
         description="Any additional metadata for this dataset",
@@ -50,13 +68,29 @@ class Datasets(Protocol):
     @webmethod(route="/datasets", method="POST")
     async def register_dataset(
         self,
-        dataset_id: str,
+        schema: Schema,
-        dataset_schema: Dict[str, ParamType],
+        uri: str,
-        url: URL,
+        uri_params: Optional[Dict[str, Any]] = None,
-        provider_dataset_id: Optional[str] = None,
-        provider_id: Optional[str] = None,
         metadata: Optional[Dict[str, Any]] = None,
-    ) -> None: ...
+        dataset_id: Optional[str] = None,
+    ) -> Dataset:
+        """
+        Register a new dataset through a file or
+
+        :param schema: The schema format of the dataset. One of
+            - jsonl_messages: The dataset is a JSONL file with messages in column format
+        :param uri: The URI of the dataset. Examples:
+           - file://mydata.jsonl
+           - s3://mybucket/myfile.jsonl
+           - https://mywebsite.com/myfile.jsonl
+           - huggingface://tatsu-lab/alpaca
+        :param uri_params: The parameters for the URI.
+           - E.g. If URL is a huggingface dataset, parameters could be uri_params={"split": "train"}
+        :param metadata: The metadata for the dataset.
+           - E.g. {"description": "My dataset"}
+        :param dataset_id: The ID of the dataset. If not provided, a random ID will be generated.
+        """
+        ...
 
     @webmethod(route="/datasets/{dataset_id:path}", method="GET")
     async def get_dataset(