fix: export StartOptions and MobileScannerViewAttributes for tests

[w3ggy]

I added StartOptions and MobileScannerViewAttributes to export to let the developers to mock the MobileScannerPlatform in tests.

I tried to write golden tests and mock the MobileScannerPlatform but I cannot do it because I cannot import classes which are necessary to override Future<MobileScannerViewAttributes> start(StartOptions startOptions)

[navaronbracke]

@w3ggy Should we mark this as @visibleForTesting ? Otherwise, I think this looks good to have.
While there aren't many people that write tests for mobile_scanner usage, there are definitely some.

See also #1042 which is also related to tests and we should look into.