Enable filtering of signal matches

Provide tooling to enable specification of pre-implemented filtering
functors for signal matches.

Add a default 'none' filter to be used when a filter isn't specified.

Change-Id: I3549d8cf44c5f475626875fa94ca3ee8f74d6d26
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/examples/match2.yaml b/examples/match2.yaml
index 3e11531..6fb9ff9 100644
--- a/examples/match2.yaml
+++ b/examples/match2.yaml
@@ -12,5 +12,7 @@
           path: /xyz/openbmc_project/testing
           interface: org.freedesktop.DBus.Properties
           member: PropertiesChanged
+      filter:
+          name: none
 
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