aiaiai: add initial support for dispatcher hook

aiaiai: add initial support for dispatcher hook

This patch adds some initial support for a dispatcher hook which could
be used to add custom headers, which will (later) be interpreted by
aiaiai to include specialized settings. A follow-on patch will add the
first initial custom header.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>