AP_Logger: add LOG_REPLAY=2 to write replay data to a separate log

[rishabsingh3003]

Summary

Add LOG_REPLAY=2 to write EKF replay data to its own log under REPLAY/, so replay capture no longer requires LOG_DISARMED or bloats the main log; compiled out by default (SITL only)

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request