[dataset] add `ValidateTlvs()`, helper methods, and unit test

[abtink]

This commit enhances dataset handling:

• Adds ValidateTlvs() which parses and validates all known TLVs within the Dataset and checks for any duplicates.
• Introduces a set of SetFrom() methods for constructing a Dataset from various types of inputs, e.g., other Dataset, TLV sequences, DatasetInfo structures, bytes read from Message. Now they consistently clear the Dataset before setting it.
• Adds WriteTlvsFrom() to update a Dataset replacing/appending a set of TLVs.
• Adds AppendTlvsFrom() to append an already encoded sequence of TLVs without validating/checking the format.
• Renames Get/SetSize() to Get/SetLength() for consistency.
• Adds test_dataset.cpp unit test for basic Dataset validation.

[size-report[bot]]

Size Report of OpenThread

Merging #10111 into main(0c6c2fedadf6516282f4430da89d2b99d551934a).

name	branch	text	data	bss	total
ot-cli-ftd	main	466840	856	66364	534060
	#10111	466952	856	66364	534172
	+/-	+112	0	0	+112
ot-ncp-ftd	main	435756	760	61576	498092
	#10111	435836	760	61576	498172
	+/-	+80	0	0	+80
libopenthread-ftd.a	main	235928	95	40310	276333
	#10111	236136	95	40310	276541
	+/-	+208	0	0	+208
libopenthread-cli-ftd.a	main	57545	0	8075	65620
	#10111	57553	0	8075	65628
	+/-	+8	0	0	+8
libopenthread-ncp-ftd.a	main	31863	0	5916	37779
	#10111	31863	0	5916	37779
	+/-	0	0	0	0
ot-cli-mtd	main	364504	760	51220	416484
	#10111	364616	760	51220	416596
	+/-	+112	0	0	+112
ot-ncp-mtd	main	347036	760	46448	394244
	#10111	347116	760	46448	394324
	+/-	+80	0	0	+80
libopenthread-mtd.a	main	157999	0	25182	183181
	#10111	158203	0	25182	183385
	+/-	+204	0	0	+204
libopenthread-cli-mtd.a	main	39787	0	8059	47846
	#10111	39787	0	8059	47846
	+/-	0	0	0	0
libopenthread-ncp-mtd.a	main	24743	0	5916	30659
	#10111	24743	0	5916	30659
	+/-	0	0	0	0
ot-cli-ftd-br	main	549976	864	131212	682052
	#10111	550072	864	131212	682148
	+/-	+96	0	0	+96
libopenthread-ftd-br.a	main	324242	100	105134	429476
	#10111	324446	100	105134	429680
	+/-	+204	0	0	+204
libopenthread-cli-ftd-br.a	main	71208	0	8099	79307
	#10111	71216	0	8099	79315
	+/-	+8	0	0	+8
ot-rcp	main	62248	564	20604	83416
	#10111	62248	564	20604	83416
	+/-	0	0	0	0
libopenthread-rcp.a	main	9542	0	5052	14594
	#10111	9542	0	5052	14594
	+/-	0	0	0	0
libopenthread-radio.a	main	18870	0	214	19084
	#10111	18870	0	214	19084
	+/-	0	0	0	0