Add indication on when the extension is active responding to a PrivateToken request

[thibmeu]

In version 3.0, where token where used, the extension icon switched from grey to orange when the website was compatible with (legacy) Privacy Pass protocol.

A similar pattern could be used for the extension when it's actively responding to a PrivateToken challenge.

This would help user get insight at whether the extension is helpful to them, as has been reported in privacypass/challenge-bypass-extension#407.