Add a Brave Flag to enable unlisted Extensions

I want to use a Extention whose isn’t listed on Google Webstore, called Blue Messenger(Banned 'cause is a Alternative to use Facebook Lite and Mobile with reactions and useful functions)

Some extentions are unavailable on Webstore nor Edge store for other reasons than Malware or privacy Issues, I was planing to move finally to brave this day, when I saw that I can’t use my fav CRX, I severally assume that I’ll not using brave till the flag is available, Sorry for bad Eng