BGP Add Path with policy limiting prefix eligibility

This feature adds support for prefix-policy, which helps BGP Add Path (BGP-AP) to identify eligible multiple path prefixes based on community values. BGP advertises these community-tagged routes in addition to the active path to a given destination. If the community value of a route does not match the community value defined in the policy, then BGP does not advertise that route. You can limit prefixes that BGP considers for multiple paths without actually knowing the prefixes in advance. Instead, a known BGP community value determines whether or not a prefix is advertised.