feat: Bring Your Own API (BYOA) (#2228)

# What does this PR do?

Prototype on a new feature to allow new APIs to be plugged in Llama
Stack. Opened for early feedback on the approach and test appetite on
the functionality.

@ashwinb @raghotham open for early feedback, thanks!

---------

Signed-off-by: Sébastien Han <seb@redhat.com>
Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.com>

llama_stack/apis/datatypes.py

@@ -4,15 +4,83 @@
 # 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 enum import Enum, EnumMeta
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 from llama_stack.schema_utils import json_schema_type
 
 
+class DynamicApiMeta(EnumMeta):
+    def __new__(cls, name, bases, namespace):
+        # Store the original enum values
+        original_values = {k: v for k, v in namespace.items() if not k.startswith("_")}
+
+        # Create the enum class
+        cls = super().__new__(cls, name, bases, namespace)
+
+        # Store the original values for reference
+        cls._original_values = original_values
+        # Initialize _dynamic_values
+        cls._dynamic_values = {}
+
+        return cls
+
+    def __call__(cls, value):
+        try:
+            return super().__call__(value)
+        except ValueError as e:
+            # If this value was already dynamically added, return it
+            if value in cls._dynamic_values:
+                return cls._dynamic_values[value]
+
+            # If the value doesn't exist, create a new enum member
+            # Create a new member name from the value
+            member_name = value.lower().replace("-", "_")
+
+            # If this member name already exists in the enum, return the existing member
+            if member_name in cls._member_map_:
+                return cls._member_map_[member_name]
+
+            # Instead of creating a new member, raise ValueError to force users to use Api.add() to
+            # register new APIs explicitly
+            raise ValueError(f"API '{value}' does not exist. Use Api.add() to register new APIs.") from e
+
+    def __iter__(cls):
+        # Allow iteration over both static and dynamic members
+        yield from super().__iter__()
+        if hasattr(cls, "_dynamic_values"):
+            yield from cls._dynamic_values.values()
+
+    def add(cls, value):
+        """
+        Add a new API to the enum.
+        Used to register external APIs.
+        """
+        member_name = value.lower().replace("-", "_")
+
+        # If this member name already exists in the enum, return it
+        if member_name in cls._member_map_:
+            return cls._member_map_[member_name]
+
+        # Create a new enum member
+        member = object.__new__(cls)
+        member._name_ = member_name
+        member._value_ = value
+
+        # Add it to the enum class
+        cls._member_map_[member_name] = member
+        cls._member_names_.append(member_name)
+        cls._member_type_ = str
+
+        # Store it in our dynamic values
+        cls._dynamic_values[value] = member
+
+        return member
+
+
 @json_schema_type
-class Api(Enum):
+class Api(Enum, metaclass=DynamicApiMeta):
     providers = "providers"
     inference = "inference"
     safety = "safety"
@@ -54,3 +122,12 @@ class Error(BaseModel):
     title: str
     detail: str
     instance: str | None = None
+
+
+class ExternalApiSpec(BaseModel):
+    """Specification for an external API implementation."""
+
+    module: str = Field(..., description="Python module containing the API implementation")
+    name: str = Field(..., description="Name of the API")
+    pip_packages: list[str] = Field(default=[], description="List of pip packages to install the API")
+    protocol: str = Field(..., description="Name of the protocol class for the API")