Add a devtools option

[kyle-ssg]

When testing features it would be useful to have the ability to see and flip flags for that instance interactively.

import FlagsmithToolbar from 'flagsmith/toolbar'
FlagsmithToolbar.init(flagsmithInstance)
FlagsmithToolbar.show()
FlagsmithToolbar.hide()

Would inject an overlay into the site where you could override any feature flag.

Notes

• We can use the generated types to allow users to select values to change the feaure to
• Users will authenticate via GitHub

[novakzaballa]

@kyle-ssg @matthewelwell Can I work on this one?