[dnssd] implement `otPlatDnssd` APIs #5155

Job	Run time
docker-check	4m 59s
buildx (latest, ubuntu:bionic, linux/amd64,linux/arm/v7,linux/arm64, yes)	2m 7s
buildx (focal, ubuntu:focal, linux/amd64,linux/arm64, yes)	2m 34s
buildx (jammy, ubuntu:jammy, linux/amd64,linux/arm64, yes)	2m 2s
buildx (reference-device, ubuntu:bionic, --build-arg REFERENCE_DEVICE=1 --build-arg BORDER_ROUTIN...	2m 4s
buildx (test, ubuntu:bionic, --build-arg OT_BACKBONE_CI=1 --build-arg REFERENCE_DEVICE=1 --build-...	2m 20s
	16m 6s

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