feat: fine grained access control policy (#2264)

This allows a set of rules to be defined for determining access to
resources. The rules are (loosely) based on the cedar policy format.

A rule defines a list of action either to permit or to forbid. It may
specify a principal or a resource that must match for the rule to take
effect. It may also specify a condition, either a 'when' or an 'unless',
with additional constraints as to where the rule applies.

A list of rules is held for each type to be protected and tried in order
to find a match. If a match is found, the request is permitted or
forbidden depening on the type of rule. If no match is found, the
request is denied. If no rules are specified for a given type, a rule
that allows any action as long as the resource attributes match the user
attributes is added (i.e. the previous behaviour is the default.

Some examples in yaml:

```
    model:
    - permit:
      principal: user-1
      actions: [create, read, delete]
      comment: user-1 has full access to all models
    - permit:
      principal: user-2
      actions: [read]
      resource: model-1
      comment: user-2 has read access to model-1 only
    - permit:
      actions: [read]
      when:
        user_in: resource.namespaces
      comment: any user has read access to models with matching attributes
    vector_db:
    - forbid:
      actions: [create, read, delete]
      unless:
        user_in: role::admin
      comment: only user with admin role can use vector_db resources
```

---------

Signed-off-by: Gordon Sim <gsim@redhat.com>

llama_stack/distribution/resolver.py

@@ -28,6 +28,7 @@ from llama_stack.apis.vector_dbs import VectorDBs
 from llama_stack.apis.vector_io import VectorIO
 from llama_stack.distribution.client import get_client_impl
 from llama_stack.distribution.datatypes import (
+    AccessRule,
     AutoRoutedProviderSpec,
     Provider,
     RoutingTableProviderSpec,
@@ -118,6 +119,7 @@ async def resolve_impls(
     run_config: StackRunConfig,
     provider_registry: ProviderRegistry,
     dist_registry: DistributionRegistry,
+    policy: list[AccessRule],
 ) -> dict[Api, Any]:
     """
     Resolves provider implementations by:
@@ -140,7 +142,7 @@ async def resolve_impls(
 
     sorted_providers = sort_providers_by_deps(providers_with_specs, run_config)
 
-    return await instantiate_providers(sorted_providers, router_apis, dist_registry, run_config)
+    return await instantiate_providers(sorted_providers, router_apis, dist_registry, run_config, policy)
 
 
 def specs_for_autorouted_apis(apis_to_serve: list[str] | set[str]) -> dict[str, dict[str, ProviderWithSpec]]:
@@ -247,6 +249,7 @@ async def instantiate_providers(
     router_apis: set[Api],
     dist_registry: DistributionRegistry,
     run_config: StackRunConfig,
+    policy: list[AccessRule],
 ) -> dict:
     """Instantiates providers asynchronously while managing dependencies."""
     impls: dict[Api, Any] = {}
@@ -261,7 +264,7 @@ async def instantiate_providers(
         if isinstance(provider.spec, RoutingTableProviderSpec):
             inner_impls = inner_impls_by_provider_id[f"inner-{provider.spec.router_api.value}"]
 
-        impl = await instantiate_provider(provider, deps, inner_impls, dist_registry, run_config)
+        impl = await instantiate_provider(provider, deps, inner_impls, dist_registry, run_config, policy)
 
         if api_str.startswith("inner-"):
             inner_impls_by_provider_id[api_str][provider.provider_id] = impl
@@ -312,6 +315,7 @@ async def instantiate_provider(
     inner_impls: dict[str, Any],
     dist_registry: DistributionRegistry,
     run_config: StackRunConfig,
+    policy: list[AccessRule],
 ):
     provider_spec = provider.spec
     if not hasattr(provider_spec, "module"):
@@ -336,13 +340,15 @@ async def instantiate_provider(
         method = "get_routing_table_impl"
 
         config = None
-        args = [provider_spec.api, inner_impls, deps, dist_registry]
+        args = [provider_spec.api, inner_impls, deps, dist_registry, policy]
     else:
         method = "get_provider_impl"
 
         config_type = instantiate_class_type(provider_spec.config_class)
         config = config_type(**provider.config)
         args = [config, deps]
+        if "policy" in inspect.signature(getattr(module, method)).parameters:
+            args.append(policy)
 
     fn = getattr(module, method)
     impl = await fn(*args)