[srp-server] allow service instance label with dot character (#9198)

This commit updates the `Srp::Server` class to correctly handle the
case where a service is registered with a dot character in its
service instance name. The first label in a service instance name is
intended as a user-friendly name and can contain dot characters
(it has fewer restrictions than other labels in a DNS name).

In particular, this commit contains the following changes:
- The `PtrRecords::ReadPtrName()` method is used in `Srp::Server` to
  read and validate the first label and the rest of the labels
  separately. This also validates the format of the parsed PTR
  record.
- The `Service::Description` class now remembers the instance label in
  addition to the full instance name. This allows the instance label
  to be easily retrieved.
- The `Dns::Name::ReadName()` method is updated to only verify that
  the labels after the first label do not contain any dot characters.
  This allows it to be used to read instance service names.
- The tests are updated to validate the behavior of the SRP server
  when the instance label contains a dot character.

tests/unit/test_srp_server.cpp

@@ -287,7 +287,7 @@ static const char kHostName[] = "myhost";
 void PrepareService1(Srp::Client::Service &aService)
 {
     static const char          kServiceName[]   = "_srv._udp";
-    static const char          kInstanceLabel[] = "srv-instance";
+    static const char          kInstanceLabel[] = "srv.instance";
     static const char          kSub1[]          = "_sub1";
     static const char          kSub2[]          = "_V1234567";
     static const char          kSub3[]          = "_XYZWS";