@abdolreza It existed as a feature on Desktop before the Android version was even created, so it is likely that it has always been a planned feature. They likely limited it to a set few search engines by default as a temporary fix as they worked to develop a better solution.
It will definitely take more than half an hour to code as it involves lots of pieces. There needs to be a settings page, a way to automatically add a search engine from a search box, a way to sync it with the Desktop version, and countless other changes in other places like the address bar. This also ignores all the testing that needs to be done to make sure it works correctly. All of which adds up to much more than half an hour...
So maybe give the devs a bit of a break. We know they want to add this feature, but we don't know what problems could be blocking its implementation or if they just haven't gotten to it yet given other priorities.