Add feature to ignore RAI Link Selection suboption for subnet selection
It seems that some vendors may not allow granular control of the option 82 suboptions which are sent. We should add a configuration parameter that allows clients to choose whether or not the RAI Link Selection suboption (option 82.5) is used as the primary source of truth for which subnet to use. Clients need to be able to choose the subnet selection logic that Kea regardless of which vendors they use for routing equipment. The specific client in question is attempting to use option 82.1 to classify packets into specific client classes, and use client classification to determine the subnet which a client has an address assigned from. The subnet specified by the routers in the Link Selection subnet is not necessarily the subnet which the client should use. The client uses Juniper routers as DHCP relays, and Juniper's docs do not shed light on how to specifically disable the Link Selection suboption: https://stage.juniper.net/documentation/us/en/software/junos/subscriber-mgmt-sessions/topics/topic-map/dhcp-option-82-using.html#id-using-dhcp-relay-agent-option-82-information
Users should be able to ignore the Link Selection suboption as a primary source of truth for subnet selection, and instead fall back to the normal subnet selection process that is used when the Link Selection suboption is not present. In this case, the routers still include a giaddr (relay address) in the bootp header that can be used for shared network selection.
(Another proposed solution was flex-option for queries)