feat(api): clean up api for evals

- as title

llama_stack/apis/datasets/datasets.py

@@ -4,19 +4,35 @@
 # 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
 
-from llama_stack.apis.common.content_types import URL
-from llama_stack.apis.common.type_system import ParamType
 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"
+
+
 class CommonDatasetFields(BaseModel):
-    dataset_schema: Dict[str, ParamType]
-    url: URL
+    schema: Schema
+    uri: str
     metadata: Dict[str, Any] = Field(
         default_factory=dict,
         description="Any additional metadata for this dataset",
@@ -50,13 +66,10 @@ class Datasets(Protocol):
     @webmethod(route="/datasets", method="POST")
     async def register_dataset(
         self,
-        dataset_id: str,
-        dataset_schema: Dict[str, ParamType],
-        url: URL,
-        provider_dataset_id: Optional[str] = None,
-        provider_id: Optional[str] = None,
+        schema: Schema,
+        uri: str,
         metadata: Optional[Dict[str, Any]] = None,
-    ) -> None: ...
+    ) -> Dataset: ...
 
     @webmethod(route="/datasets/{dataset_id:path}", method="GET")
     async def get_dataset(