res/res_endpoint_stats: Add module to emit endpoint StatsD statistics

This patch adds a module that emits StatsD statistics about Asterisk
endpoints. This includes:
 * A GAUGE statistic for endpoint states, tracking how many endpoints are in
   a particular state.
 * A GAUGE statistic for each endpoint, counting the number of channels
   currently associated with an endpoint.

ASTERISK-25572

Change-Id: If7e1333c5aeda8d136850b30c2101c0ee1c97305