[mdns] add support for advertising subtypes

This commit adds support of advertising and discovering services
with subtypes.

With this change, client can publish a new service with subtypes with
code:
```
NsdServiceInfo info = new NsdServiceInfo();
info.setServiceName("My Service");
info.setServiceType("_meshcop._udp");
info.setSubtypes(Set.of("_abc"));

nsdManager.registerService(info, PROTOCOL_DNS_SD, listener);
```

Bug: 265095929
Test: atest CtsNetTestCases FrameworksNetTests
Change-Id: Ia83182e2d43002243d1ef4357d14a8056d7da14b