[net] SRP client implementation (#6038)

This commit adds support for SRP (Service Registration Protocol)
client in OpenThread. The implementation allows a user to provide host
info (host name and a list of host IPv6 addresses) along with a list
of services to be registered with an SRP server. Services and/or host
addresses can be added or removed during operation of client.

Users can get the list of services and host info and their current
state (indicating, for example, if service is registered with server,
being registered or being removed, etc). Users can also provide a
callback to get notified whenever there is change or an error.

When there is a new request (e.g., a new service is added/removed)
that requires an update, the SRP client will wait for a short delay
before preparing and sending an SRP update message to server. This
allows user to provide more changes that are then all sent in the same
update message. The implementation handles retries in case of
different errors and failures. An exponentially increasing retry wait
interval (with configurable min, max, and growth factor) is
implemented.

The implementation also manages the lease renew time for each service
and refreshes (re-registers) services with server before lease is
expired. It supports "opportunistic early refresh" mechanism such that
when sending an SRP update, the services that are not yet expired but
are close, are allowed to refresh early and are included in the SRP
update. This helps place more services on the same lease refresh
schedule reducing number of messages sent to the SRP server. This
behavior (whether to allow early refresh or not, and its related
parameters) can be controlled through a set of OT config definitions.

src/core/common/notifier.cpp

@@ -183,6 +183,9 @@ void Notifier::EmitEvents(void)
 #if OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE
     Get<BorderRouter::RoutingManager>().HandleNotifierEvents(events);
 #endif
+#if OPENTHREAD_CONFIG_SRP_CLIENT_ENABLE
+    Get<Srp::Client>().HandleNotifierEvents(events);
+#endif
 
     for (ExternalCallback &callback : mExternalCallbacks)
     {