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/distribution/external.py

@@ -0,0 +1,54 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+
+import yaml
+
+from llama_stack.apis.datatypes import Api, ExternalApiSpec
+from llama_stack.distribution.datatypes import BuildConfig, StackRunConfig
+from llama_stack.log import get_logger
+
+logger = get_logger(name=__name__, category="core")
+
+
+def load_external_apis(config: StackRunConfig | BuildConfig | None) -> dict[Api, ExternalApiSpec]:
+    """Load external API specifications from the configured directory.
+
+    Args:
+        config: StackRunConfig or BuildConfig containing the external APIs directory path
+
+    Returns:
+        A dictionary mapping API names to their specifications
+    """
+    if not config or not config.external_apis_dir:
+        return {}
+
+    external_apis_dir = config.external_apis_dir.expanduser().resolve()
+    if not external_apis_dir.is_dir():
+        logger.error(f"External APIs directory is not a directory: {external_apis_dir}")
+        return {}
+
+    logger.info(f"Loading external APIs from {external_apis_dir}")
+    external_apis: dict[Api, ExternalApiSpec] = {}
+
+    # Look for YAML files in the external APIs directory
+    for yaml_path in external_apis_dir.glob("*.yaml"):
+        try:
+            with open(yaml_path) as f:
+                spec_data = yaml.safe_load(f)
+
+            spec = ExternalApiSpec(**spec_data)
+            api = Api.add(spec.name)
+            logger.info(f"Loaded external API spec for {spec.name} from {yaml_path}")
+            external_apis[api] = spec
+        except yaml.YAMLError as yaml_err:
+            logger.error(f"Failed to parse YAML file {yaml_path}: {yaml_err}")
+            raise
+        except Exception:
+            logger.exception(f"Failed to load external API spec from {yaml_path}")
+            raise
+
+    return external_apis