util: Add FormatISO8601Time function for time-only ISO format

Adds a new utility function FormatISO8601Time that formats a Unix timestamp
into an ISO 8601 time string (HH:MM:SSZ). This complements the existing
FormatISO8601DateTime and FormatISO8601Date functions, providing a way to
format just the time component of a timestamp.

The function uses C++20 chrono features to handle time calculations and
formatting, maintaining consistency with the existing ISO 8601 formatting
functions.

Author: rebroad

src/util/time.cpp

@@ -75,6 +75,14 @@ void MockableSteadyClock::ClearMockTime()
 
 int64_t GetTime() { return GetTime<std::chrono::seconds>().count(); }
 
+std::string FormatISO8601Time(int64_t nTime)
+{
+    const std::chrono::sys_seconds secs{std::chrono::seconds{nTime}};
+    const auto days{std::chrono::floor<std::chrono::days>(secs)};
+    const std::chrono::hh_mm_ss hms{secs - days};
+    return strprintf("%02i:%02i:%02iZ", hms.hours().count(), hms.minutes().count(), hms.seconds().count());
+}
+
 std::string FormatISO8601DateTime(int64_t nTime)
 {
     const std::chrono::sys_seconds secs{std::chrono::seconds{nTime}};

src/util/time.h

@@ -130,6 +130,7 @@ T GetTime()
  * ISO 8601 formatting is preferred. Use the FormatISO8601{DateTime,Date}
  * helper functions if possible.
  */
+std::string FormatISO8601Time(int64_t nTime);
 std::string FormatISO8601DateTime(int64_t nTime);
 std::string FormatISO8601Date(int64_t nTime);
 std::optional<int64_t> ParseISO8601DateTime(std::string_view str);