caveats: add an unsigned_accessibility caveat message

This is useful for applications that are not signed by the developer and
require Accessibility access.

Because the app is not signed, macOS only authorizes the current binary,
and so when it is updated (and the binary changes) the new version is
unsigned, despite the app still showing as ticked in System Preferences.
The user has to manually untick and retick the app each time.

The ideal fix is for the developer to sign their app, but not all
developers are willing to pay for this, so the best we can do is to
advise users of the workaround/solution.

Refs: https://github.com/Homebrew/homebrew-cask/pull/83157