[routing-manager] track and report last update time of routers

[abtink]

This commit enhances the DiscoveredPrefixTable::Router class to track the last time an ND6 message (RA or NS) was received from each discovered router. This information is now available through the otBorderRoutingRouterEntry and can be viewed using the br routers CLI command.

[size-report]

Size Report of OpenThread

Merging #10244 into main(0ce49fc).

name	branch	text	data	bss	total
ot-cli-ftd	main	467056	856	66364	534276
	#10244	467056	856	66364	534276
	+/-	0	0	0	0
ot-ncp-ftd	main	435900	760	61576	498236
	#10244	435900	760	61576	498236
	+/-	0	0	0	0
libopenthread-ftd.a	main	236174	95	40310	276579
	#10244	236174	95	40310	276579
	+/-	0	0	0	0
libopenthread-cli-ftd.a	main	57533	0	8075	65608
	#10244	57533	0	8075	65608
	+/-	0	0	0	0
libopenthread-ncp-ftd.a	main	31863	0	5916	37779
	#10244	31863	0	5916	37779
	+/-	0	0	0	0
ot-cli-mtd	main	364520	760	51220	416500
	#10244	364520	760	51220	416500
	+/-	0	0	0	0
ot-ncp-mtd	main	347060	760	46448	394268
	#10244	347060	760	46448	394268
	+/-	0	0	0	0
libopenthread-mtd.a	main	158173	0	25182	183355
	#10244	158173	0	25182	183355
	+/-	0	0	0	0
libopenthread-cli-mtd.a	main	39756	0	8059	47815
	#10244	39756	0	8059	47815
	+/-	0	0	0	0
libopenthread-ncp-mtd.a	main	24743	0	5916	30659
	#10244	24743	0	5916	30659
	+/-	0	0	0	0
ot-cli-ftd-br	main	550696	864	131204	682764
	#10244	550792	864	131204	682860
	+/-	+96	0	0	+96
libopenthread-ftd-br.a	main	324648	100	105126	429874
	#10244	324702	100	105126	429928
	+/-	+54	0	0	+54
libopenthread-cli-ftd-br.a	main	71320	0	8099	79419
	#10244	71379	0	8099	79478
	+/-	+59	0	0	+59
ot-rcp	main	62216	564	20604	83384
	#10244	62216	564	20604	83384
	+/-	0	0	0	0
libopenthread-rcp.a	main	9542	0	5052	14594
	#10244	9542	0	5052	14594
	+/-	0	0	0	0
libopenthread-radio.a	main	18870	0	214	19084
	#10244	18870	0	214	19084
	+/-	0	0	0	0