[dataset-manager] track local dataset in DatasetManager

[abtink]

This commit updates the DatasetManager to track information about the local dataset, such as the local timestamp, update time, and whether it is saved in non-volatile settings. This change effectively merges the DatasetLocal logic into DatasetManager, allowing for the removal of DatasetLocal and simplifying the code.

[size-report]

Size Report of OpenThread

Merging #10275 into main(10ccec2).

name	branch	text	data	bss	total
ot-cli-ftd	main	467056	856	66364	534276
	#10275	466984	856	66348	534188
	+/-	-72	0	-16	-88
ot-ncp-ftd	main	435900	760	61576	498236
	#10275	435820	760	61560	498140
	+/-	-80	0	-16	-96
libopenthread-ftd.a	main	236174	95	40310	276579
	#10275	236111	95	40294	276500
	+/-	-63	0	-16	-79
libopenthread-cli-ftd.a	main	57533	0	8075	65608
	#10275	57533	0	8075	65608
	+/-	0	0	0	0
libopenthread-ncp-ftd.a	main	31863	0	5916	37779
	#10275	31863	0	5916	37779
	+/-	0	0	0	0
ot-cli-mtd	main	364520	760	51220	416500
	#10275	364488	760	51204	416452
	+/-	-32	0	-16	-48
ot-ncp-mtd	main	347060	760	46448	394268
	#10275	347020	760	46432	394212
	+/-	-40	0	-16	-56
libopenthread-mtd.a	main	158173	0	25182	183355
	#10275	158130	0	25166	183296
	+/-	-43	0	-16	-59
libopenthread-cli-mtd.a	main	39756	0	8059	47815
	#10275	39756	0	8059	47815
	+/-	0	0	0	0
libopenthread-ncp-mtd.a	main	24743	0	5916	30659
	#10275	24743	0	5916	30659
	+/-	0	0	0	0
ot-cli-ftd-br	main	550696	864	131204	682764
	#10275	550536	864	131188	682588
	+/-	-160	0	-16	-176
libopenthread-ftd-br.a	main	324648	100	105126	429874
	#10275	324515	100	105110	429725
	+/-	-133	0	-16	-149
libopenthread-cli-ftd-br.a	main	71320	0	8099	79419
	#10275	71320	0	8099	79419
	+/-	0	0	0	0
ot-rcp	main	62216	564	20604	83384
	#10275	62216	564	20604	83384
	+/-	0	0	0	0
libopenthread-rcp.a	main	9542	0	5052	14594
	#10275	9542	0	5052	14594
	+/-	0	0	0	0
libopenthread-radio.a	main	18870	0	214	19084
	#10275	18870	0	214	19084
	+/-	0	0	0	0