_context.py: Added get_artifact_cache_usage()

A frontend facing API for obtaining usage statistics.

I would have put this on Stream instead, but the Context
seems to be the de facto place for looking up the artifact cache
in general so let's put it here.

buildstream/_context.py

@@ -30,7 +30,7 @@ from . import _yaml
 from ._exceptions import LoadError, LoadErrorReason, BstError
 from ._message import Message, MessageType
 from ._profile import Topics, profile_start, profile_end
-from ._artifactcache import ArtifactCache
+from ._artifactcache import ArtifactCache, ArtifactCacheUsage
 from ._cas import CASCache
 from ._workspaces import Workspaces, WorkspaceProjectCache, WORKSPACE_PROJECT_FILE
 from .plugin import _plugin_lookup
@@ -289,6 +289,16 @@ class Context():
 
         return self._artifactcache
 
+    # get_artifact_cache_usage()
+    #
+    # Fetches the current usage of the artifact cache
+    #
+    # Returns:
+    #     (ArtifactCacheUsage): The current status
+    #
+    def get_artifact_cache_usage(self):
+        return ArtifactCacheUsage(self.artifactcache)
+
     # add_project():
     #
     # Add a project to the context.