Sandbox: Add annotations to CreateSandbox surface

An oft employed scheme for a lot of our APIs is to include an
annotations field which is just a map of string to string. This
usually allows folks using the API to send over metadata or auxiliary
information without needing to get a new field added (especially where
the field might not make sense for it to be a standalone field). I think
having annotations for CreateSandbox make sense for this same use case.

Signed-off-by: Danny Canter <danny@dcantah.dev>

api/runtime/sandbox/v1/sandbox.proto

@@ -67,6 +67,7 @@ message CreateSandboxRequest {
 	repeated containerd.types.Mount rootfs = 3;
 	google.protobuf.Any options = 4;
 	string netns_path = 5;
+	map<string, string> annotations = 6;
 }
 
 message CreateSandboxResponse {}