Update allowed URL in WebView

[imSaharukh]

Fixes issue #12.

Problem

The issue was identified as a result of this reported problem, where the WebView was encountering a blank white screen on iOS. The root cause was traced back to the incorrect handling of URLs in the code.

Solution

The proposed solution corrects the URL validation logic. Previously, the code used request.url.startsWith("https://www.bkash.com/"), which proved to be incorrect. The fix now considers that sandbox URLs start with https://sandbox.payment.bkash.com, and live URLs start with https://payment.bkash.com.

Changes Made

• Updated the URL validation logic in flutter_bkash_view.dart.