[routing-manager] stop PD if there is another prefix with higher preference in netdata (#10289)

This commit adds a new IDLE state to PdPrefixManager.

PdPrefixManager enters idle state when PD is enabled and there is
already a BR requesting PD prefix. When there are multiple BRs
publishing PD prefix at the same time, the one with lexcial smaller
prefix wins.

script/test

@@ -353,6 +353,7 @@ do_build_otbr_docker()
         "-DOT_UPTIME=ON"
         "-DOTBR_DNS_UPSTREAM_QUERY=ON"
         "-DOTBR_DUA_ROUTING=ON"
+        "-DOTBR_DHCP6_PD=ON"
     )
     local args=(
         "BORDER_ROUTING=${BORDER_ROUTING}"