[trel] manage mDNS/DNSSD and peer discovery in core (#11528)

This commit enhances the TREL module to manage mDNS/DNSSD service
registration and peer discovery (browse and resolving for TREL
services). This feature can be controlled through
`OPENTHREAD_CONFIG_TREL_MANAGE_DNSSD_ENABLE` (and/or the CMake option
`OT_TREL_MANAGE_DNSSD`).

When enabled, TREL will utilize the `Dnssd` module, which provides
mDNS-related APIs. This can be tied to OpenThread's native mDNS
implementation or to `otPlatDnssd` (i.e., provided by the platform
layer).

This commit also adds support for the TREL platform in the `Nexus`
test framework and uses this to add a detailed `test_trel` case. This
test covers basic TREL peer discovery and operation, along with
specific scenarios such as peer removal delay, delayed mDNS start,
TREL service name conflict resolution, host address changes, and
supporting multiple services on the same host (while unlikely in
actual deployments, this can be useful for testing and simulation
where a single machine may act as multiple Thread nodes, thus
advertising multiple TREL services from the same hostname. This is
explicitly supported by the implementation and covered in the
tests).

etc/cmake/options.cmake

@@ -255,6 +255,7 @@ ot_option(OT_SRP_SERVER_FAST_START_MDOE OPENTHREAD_CONFIG_SRP_SERVER_FAST_START_
 ot_option(OT_TCP OPENTHREAD_CONFIG_TCP_ENABLE "TCP")
 ot_option(OT_TIME_SYNC OPENTHREAD_CONFIG_TIME_SYNC_ENABLE "time synchronization service")
 ot_option(OT_TREL OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE "TREL radio link for Thread over Infrastructure feature")
+ot_option(OT_TREL_MANAGE_DNSSD OPENTHREAD_CONFIG_TREL_MANAGE_DNSSD_ENABLE "TREL to manage DNSSD and peer discovery")
 ot_option(OT_TX_BEACON_PAYLOAD OPENTHREAD_CONFIG_MAC_OUTGOING_BEACON_PAYLOAD_ENABLE "tx beacon payload")
 ot_option(OT_TX_QUEUE_STATS OPENTHREAD_CONFIG_TX_QUEUE_STATISTICS_ENABLE "tx queue statistics")
 ot_option(OT_UDP_FORWARD OPENTHREAD_CONFIG_UDP_FORWARD_ENABLE "UDP forward")