Achievements: Separate token into its own secrets.ini file

[RedPanda4552]

Description of Changes

Creates a separate secrets.ini file for storing sensitive or privileged information. Adds the RetroAchievements login token to this file, removing it from the base PCSX2.ini.

Please give this one a very thorough grilling on code review, I do not know if I did this right, I probably have some kind of unsafe access somewhere, something something threading and locks and internal vs external core/host stuff.

Rationale behind Changes

Makes it safer to share actual settings ini files without potentially compromising RetroAchievements sessions tokens.

Suggested Testing Steps

Try logging in to RetroAchievements, verify your login is respected across restarts, check if your token is still present in your old PCSX2.ini file (this one might need a separate action to remove but I'm not sure how we go about that, tbd).

Did you use AI to help find, test, or implement this issue or feature?

No, and I'm not angry, just disappointed that we have to ask people.

[chaoticgd]

Not a thorough review, just some stuff that I noticed.

[RedPanda4552]

Made some adjustments per chaotic's feedback. Token should now remove itself from the main file if already present, secrets.ini should auto generate when it does so on boot. Also fixed the function mixup, added timer and mutex, and generally made it a bit cleaner, and I think properly locking said mutex now.