bgpd: abstract ecom into struct for l3 route targets

bgpd: abstract ecom into struct for l3 route targets

Abstract the ecommunity into a container struct for L3
route targets so that we can add some additional info
via flags to go along with RT configs without modifying
the used elsewhere ecommunity struct. This functions as a
wrapper everywhere its used including the import/export lists.
The flags will be used in later commits to change behavior
when importing/exporting routes.

Signed-off-by: Stephen Worley <sworley@nvidia.com>