Add support to write ELT log on standard output (stdout)

Add event_log_track_stdout sub-module : Provide support to write ELT log on php://stdout or php://stderr for better log handling with Docker. See modules/event_log_track_stdout/README.md
hook_token_info() and hook_tokens() are moved from the event_log_track_syslog module to the event_log_track module. This is to mutualize the code used by the event_log_track_syslog and event_log_track_stdout sub-modules.
hook_event_log_track_alter() is triggered earlier in the event_log_track_insert() function. This allows other modules to modify the log before it is written, regardless of its destination (syslog, stdout or database).
The schema of the event_log_track.settings config is modified : Items concerning syslog and stdout are grouped in mapping object. The event_log_track_update_9405() method is used to recover existing syslog configurations.

Issue #3449312: Add support stdout