acl-plugin: implement ACL lookup contexts for "ACL as a service" use by other plugins

acl-plugin: implement ACL lookup contexts for "ACL as a service" use by other plugins

This code implements the functionality required for other plugins wishing
to perform ACL lookups in the contexts of their choice, rather than only
in the context of the interface in/out.

The lookups are the stateless ACLs - there is no concept of "direction"
within the context, hence no concept of "connection" either.

The plugins need to include the

The file acl_lookup_context.md has more info.

Change-Id: I91ba97428cc92b24d1517e808dc2fd8e56ea2f8d
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>