contrib/exporters/core: Populate the X-Websocket-Namespace header
contrib/exporters/core: Populate the X-Websocket-Namespace header

Following changes in the Skydive flow server to publish flows by capture
ID, the exporter subscription didn't receive any websocket messages it didn't
supply any namespace during connection (and therefore got a default
WildcardNamespace = `"*"`).  The flow server looks for subscribers
literally according to their requested namespace (and didn't take `"*"`
into account).

This fix makes the exporter websocket subscriber populate the
`X-Websocket-Namespace` HTTP header (on the websocket connection) with
the namespace `flow` or the namespace `flow/<CAPTURE_ID>` (if configured
to subscribe to a specific capture ID).

skydive-project/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive