This commit implements ncp version of otPlatDnssdRegisterHost.
1. move the position of `SPINEL_PROP_DNSSD_STATE` in `spinel.h`
because it was by mistake put within the range of
`SPINEL_PROP_SRP_SERVER`. The value is unchanged.
2. A template method `DnssdUpdate` is added because the same process
works also for DNSSD service and key record which will be added in
following PRs.
This commit implements NCP based version of `otPlatDnssdGetState`.
A unit test is added to verify the implementation. This commit also
adjusts the unit test workflow to put the ncp unit tests into a
separate item. The direct reason for this commit is that `otPlatDnssd`
APIs haven't been implemented in `FakePlatform` and when
`OT_PLATFORM_DNSSD` is turned, the build failed. Let's first run NCP
unit tests separately and later try to merge the test items.
Li Cao