[srp-server] add support for service subtypes (#6760)

This commit adds support for service subtypes in SRP server. It
updates the internal data model to store services in `Srp::Server`.
Every `Host` now has a list of `Service` entries along with a list of
`Service::Description` entries. These types mirror the SRP update
message format and the set of instructions that form the SRP message.
The `Service` entries represent the "Service Discovery Instructions",
i.e., the PTR records mapping a service name or a subtype name to a
service instance. A `Service::Description` entry represents the the
SRV and TXT records. A `Service` entry is always associated with a
`Service::Description` and the subtypes of the same service instance
all share the same `Service::Description` entry.

This commit also adds a new method `Host::FindNextService()` and the
public API `otSrpServerHostFindNextService()` which is very flexible
and can be used in different ways. It can be used to iterate over the
full list of services, or over a specific subset of services matching
certain conditions, e.g., iterate over all base services excluding
subtypes, or over all subtypes of an instance, or over all deleted
services, etc. It can also be used to find a specific service with a
given instance and service names.

This commit also simplifies and enhances the logging in `Srp::Server`.
In particular, when a new host is added, we now also log the list of
services being added along with it. Also a change to a `Service` is
only logged if the `Service` is marked as committed. This ensures
that temporary `Service` entries associated with a newly received SRP
update message are not logged (e.g., when an associated temporary
`Host` object is being freed after its content is merged with an
existing `Host` entry).

Finally, this commit adds a test `test_srp_sub_type.py` to cover the
subtype service registration on SRP client and server.

tools/otci/otci/otci.py

@@ -918,6 +918,8 @@ class OTCI(object):
 
                     v = v[1:-1]
                     info['addresses'] = list(map(Ip6Addr, v.split(', ')))
+                elif k == 'subtypes':
+                    info[k] = list() if v == '(null)' else list(v.split(','))
                 elif k in ('port', 'weight', 'priority'):
                     info[k] = int(v)
                 elif k in ('host',):