[border-agent] manage mDNS MeshCoP service registrations (#11455)

This commit updates `BorderAgent` modules to directly manage the
registration of mDNS MeshCoP services. Previously, this was the
responsibility of the platform or higher-level code. This behavior is
enabled using `OPENTHREAD_CONFIG_BORDER_AGENT_MESHCOP_SERVICE_ENABLE`
configuration option.

When enabled, the `BorderAgent` module itself will register the
`_meshcop._udp` service name with properly formatted TXT data. As the
state changes, the service registration is updated accordingly.

If the ephemeral key feature is enabled and used, the `BorderAgent`
will also manage the registration of the `_meshcop-e._udp` service.

The implementation allows the service instance name to be configured
in different ways. The Thread specification recommends using a
user-friendly name, such as "<VendorName> <ProductName>". The
name can be set using a newly added configuration option, or
alternatively, using a newly added public API for projects where the
name needs to be set at run-time after device initialization.

This commit also updates `test_border_agent`, validating all the
newly added behaviors related to MeshCoP service registrations.

tests/unit/test_mdns.cpp

@@ -1849,6 +1849,14 @@ Core *InitTest(void)
 
     VerifyOrQuit(sInstance != nullptr);
 
+#if OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE
+    // Disable the Border Agent to prevent its attempt to
+    // register the `_meshcop._udp` service from
+    // interfering with this test.
+
+    sInstance->Get<MeshCoP::BorderAgent>().SetEnabled(false);
+#endif
+
     return &sInstance->Get<Core>();
 }