Add metadata to interfaces

Signed-off-by: Michael Zappa <michaelzappa@microsoft.com>

SPEC.md

@@ -568,6 +568,7 @@ Plugins must output a JSON object with the following keys upon a successful `ADD
     - `name`: The name of the interface.
     - `mac`: The hardware address of the interface (if applicable).
     - `sandbox`: The isolation domain reference (e.g. path to network namespace) for the interface, or empty if on the host. For interfaces created inside the container, this should be the value passed via `CNI_NETNS`.
+    - `metadata`: a dictionary consisting of interface configurion information.
 - `ips`: IPs assigned by this attachment. Plugins may include IPs assigned external to the container.
     - `address` (string): an IP address in CIDR notation (eg "192.168.1.3/24").
     - `gateway` (string): the default gateway for this subnet, if one exists.