Optimize compile times by isolating chrono usage

- move chrono implementation details out of the public Clock header
  - define Sys system time as Unix epoch milliseconds
  - add getUnixTimestamp() for epoch timestamps in seconds
  - replace chrono wall-clock usage in ecode plugins with Sys APIs
  - update existing Sys time callers for the new millisecond contract
  - remove redundant chrono inclusion from HTTP tests
  - remove the completed user-defined snippets plan

This prevents chrono and std::format internals from being instantiated across large portions of the project while preserving timestamp precision.

include/eepp/system/clock.hpp

@@ -1,7 +1,7 @@
 #ifndef EE_SYSTEMCTIMER_H
 #define EE_SYSTEMCTIMER_H
 
-#include <chrono>
+#include <eepp/config.hpp>
 #include <eepp/system/time.hpp>
 
 namespace EE { namespace System {
@@ -25,11 +25,7 @@ class EE_API Clock {
 	Time getElapsedTimeAndReset();
 
   private:
-	using ClockImpl =
-		std::conditional_t<std::chrono::high_resolution_clock::is_steady,
-						   std::chrono::high_resolution_clock, std::chrono::steady_clock>;
-
-	ClockImpl::time_point mRefPoint{ ClockImpl::now() }; //!< Time of last reset
+	Int64 mRefPoint{ 0 }; //!< Time of last reset in microseconds
 };
 
 }} // namespace EE::System

include/eepp/system/sys.hpp

@@ -57,8 +57,11 @@ class EE_API Sys {
 	/** @return The process path ( the executable file path ) */
 	static std::string getProcessFilePath();
 
-	/** @return The System Time */
-	static double getSystemTime();
+	/** @return The Unix epoch time in milliseconds. */
+	static Int64 getSystemTime();
+
+	/** @return The Unix epoch time in seconds. */
+	static Int64 getUnixTimestamp();
 
 	/** @return The OS Name
 	 *  @param showReleaseName Instead of returning only the OS Name, it will append the release