Change: adds a Lambda scanner for easier testing

[nichtsfrei]

Instead of having to copy and paste the async traits over and over for
testing purposes a new Lambda scanner is created.

This allows us to easily build a scanner based on sync functions.

To use it you can use the LambdaBuilder:

use models::scanner::Error as ScanError;
use models::scanner::LambdaBuilder;
let builder = LambdaBuilder::default().with_start(|_|
Err(ScanError::Unexpected("meh".to_string())));
let scanner = builder.build();