Add a per eBGP session role to the config.

Add a per eBGP session role to the config.

This somewhat replaces the RFC 9234 open policy role. This is done because
ASPA requires the same role to be present to properly validate paths.
For iBGP sessions the role is forced to ROLE_NONE. If no role is set on
an ebgp session then 'announce policy' is forced to 'no'.
Also make sure the the role capability is only added if the role is set.
OK tb@