NetworkManager does not provide a ready way to accept Router Advertisements when using DHCPv6

Issue

• Would like to use DHCPv6 and accept Router Advertisements when using NetworkManager.
• Setting the ipv6.method of a NetworkManager (NM) connection to dhcp causes NM to turn off the kernel sysctl net.ipv6.conf.<interface>.accept_ra so the host ignores IPv6 Router Advertisements (RAs) and then never configures a default route.