[sntp] clean up and improve Sntp::Client (#13114)

This commit refactors and improves the `Sntp::Client` class by
adopting common OpenThread patterns and simplifying the logic.

Key changes:
- Introduced `Sntp::Client::QueryInfo` core class to wrap the
  public `otSntpQuery` structure.
- Added `Timestamp` class to handle SNTP timestamps, simplifying
  the `Header` structure.
- Renamed methods and variables to be more concise and consistent
  (e.g., `FinalizeSntpTransaction` to `Finalize`,
  `mRetransmissionTimer` to `mTimer`).
- Simplified the `HandleUdpReceive` logic by splitting response
  processing into `ProcessResponse`.

This change improves code readability and maintainability of the
SNTP client module.

tests/toranj/openthread-core-toranj-config.h

@@ -199,6 +199,8 @@
 
 #define OPENTHREAD_CONFIG_SRP_CLIENT_AUTO_START_DEFAULT_MODE 0
 
+#define OPENTHREAD_CONFIG_SNTP_CLIENT_ENABLE 1
+
 #define OPENTHREAD_CONFIG_HISTORY_TRACKER_ENABLE 1
 
 #define OPENTHREAD_CONFIG_DNSSD_SERVER_ENABLE 1