Add addOptionalHostPermissions

[yescallop]

In order to solve huhu/rust-search-extension#263, we need to make the "Allow access to file URLs" setting show. I suggest we use this solution:

I ended up adding "optional_host_permissions": [ "file:///*" ]
This way the "Allow access to file URLs" is shown, navigation works, but I don't actually get read permission.

The next part of the fix is at huhu/rust-search-extension#275. Thank you for your attention.

[Folyd]

LGTM, thanks