[mdns] add APIs to iterate over browsers and resolvers

[abtink]

This commit adds APIs to iterate over mDNS browsers and resolvers, along with related CLI commands. These are intended for testing. It also introduces a build config ENTRY_ITERATION_API_ENABLE to control whether the mDNS module provides mechanisms and public APIs for entry iteration.

[size-report]

Size Report of OpenThread

Merging #10140 into main(59e202c).

name	branch	text	data	bss	total
ot-cli-ftd	main	466656	856	66332	533844
	#10140	466656	856	66332	533844
	+/-	0	0	0	0
ot-ncp-ftd	main	435508	760	61544	497812
	#10140	435508	760	61544	497812
	+/-	0	0	0	0
libopenthread-ftd.a	main	235839	95	40278	276212
	#10140	235839	95	40278	276212
	+/-	0	0	0	0
libopenthread-cli-ftd.a	main	57589	0	8075	65664
	#10140	57589	0	8075	65664
	+/-	0	0	0	0
libopenthread-ncp-ftd.a	main	31863	0	5916	37779
	#10140	31863	0	5916	37779
	+/-	0	0	0	0
ot-cli-mtd	main	364560	760	51204	416524
	#10140	364560	760	51204	416524
	+/-	0	0	0	0
ot-ncp-mtd	main	347044	760	46432	394236
	#10140	347044	760	46432	394236
	+/-	0	0	0	0
libopenthread-mtd.a	main	157956	0	25166	183122
	#10140	157956	0	25166	183122
	+/-	0	0	0	0
libopenthread-cli-mtd.a	main	39812	0	8059	47871
	#10140	39812	0	8059	47871
	+/-	0	0	0	0
libopenthread-ncp-mtd.a	main	24743	0	5916	30659
	#10140	24743	0	5916	30659
	+/-	0	0	0	0
ot-cli-ftd-br	main	550000	864	131172	682036
	#10140	550000	864	131172	682036
	+/-	0	0	0	0
libopenthread-ftd-br.a	main	323816	100	105094	429010
	#10140	323148	100	105094	428342
	+/-	-668	0	0	-668
libopenthread-cli-ftd-br.a	main	71450	0	8099	79549
	#10140	71450	0	8099	79549
	+/-	0	0	0	0
ot-rcp	main	62280	564	20604	83448
	#10140	62280	564	20604	83448
	+/-	0	0	0	0
libopenthread-rcp.a	main	9542	0	5052	14594
	#10140	9542	0	5052	14594
	+/-	0	0	0	0
libopenthread-radio.a	main	19042	0	214	19256
	#10140	19042	0	214	19256
	+/-	0	0	0	0