fix: Pass external_config_dir to BuildConfig (#2190)

# What does this PR do?

The `external_config_dir` configuration parameter is not being passed to
the `BuildConfig` for `LlamaStackAsLibraryClient`.

This prevents _plugin_ providers from being loaded when `llama-stack` is
uses as a library.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
I ran `LlamaStackAsLibraryClient` with a configuration file that
contained `external_config_dir` and related configuration.

It does not work without this change: _external_ providers are not
resolved.

It does work with this change 👍 

[//]: # (## Documentation)

llama_stack/distribution/datatypes.py

@@ -340,8 +340,17 @@ class BuildConfig(BaseModel):
         default=None,
         description="Name of the distribution to build",
     )
-    external_providers_dir: str | None = Field(
+    external_providers_dir: Path | None = Field(
         default=None,
         description="Path to directory containing external provider implementations. The providers packages will be resolved from this directory. "
         "pip_packages MUST contain the provider package name.",
     )
+
+    @field_validator("external_providers_dir")
+    @classmethod
+    def validate_external_providers_dir(cls, v):
+        if v is None:
+            return None
+        if isinstance(v, str):
+            return Path(v)
+        return v

llama_stack/distribution/library_client.py

@@ -226,6 +226,7 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
                     distribution_spec=DistributionSpec(
                         providers=provider_types,
                     ),
+                    external_providers_dir=self.config.external_providers_dir,
                 )
                 print_pip_install_help(build_config)
             else: