Support for several apps added including verification of end-to-end
working.

Apps found by searching for "BHIM" and "UPI" on play store and app
store and also from these links:

https://www.upichalega.com/upi-partners.php
https://www.npci.org.in/upi-PSP&3rdpartyApps
https://www.npci.org.in/upi-live-members
https://www.sebi.gov.in/sebiweb/other/OtherAction.do?doRecognisedFpi=yes&intmId=43

Main repo merge

iOS Support
- iOS method channel providing canLaunch and launch methods towards
  discovering UPI apps and launching them respectively.
- Added the custom schemes of the iOS apps in the example app's
  Info.plist to make iOS payment work in the example app.
- Changed the example app to show different views for Android and
  iOS. Android view is same as before. iOS view clearly shows that
  none of the apps can be picked by the user, as on iOS they are
  picked by iOS against the upi://pay? calls made to make the UPI
  transaction. The iOS view also shows that some of the apps that
  cannot be discovered as they don't implement any unique custom
  scheme may be invoked by iOS.
- Fixed the issues that were preventing the build for iOS. Example
  domain was one such aspect that was fixed in Android app for the
  example, too. Recreation of the package was used to fix some.
- Added logo images for iOS apps that implement 'upi' custom
  scheme and are launchable hence

Multiple apps support
- Play Store and App Store were searched to find an almost
  exhaustive list of apps and they were played with to figure
  out their current status of working. All the apps are added
  to the package along with their identity and status information.

Package behaviour and presentation changes
- Earlier, this package looked to assume ownership of UPI app
  functionality/working. However; with changing/evolving UPI apps
  ecosystem, this needs to be incorporated periodically.
- All the dynamics of the UPI apps' behaviour is now captured in
  the package, and though the default behaviour of the package is
  to present only the completely successfully working apps to the
  user, now the user can see all supported by the package currently
  if he/she requires to experiment for themselves. This avoids
  repetitive UPI apps verification work across the dev community.

Android Intent cleanups
- For discovering, the intent call should just resemble upi://pay?.
  There is no need for dummy parameters.
- Removed the commented out version of earlier intent preparation
  logic

Restructuring
- Existing structure changed such that upi_pay.dart serves as
  library wrapper only, exporting relevant modules, and existing
  and new code are moved into multiple unit modules, towards
  more maintainable code

Docs
- Added comments in public APIs, classes and enums and generated
  doc using dartdoc

Cleanups/Improvements
- Dependencies cleaned up in pubspec.yaml and moved to latest

UPI Applications Status README
- Contains the application status records as per the verifications
  done during April, 2021 (ended 19-Apr-2021)

Added new screen capture GIFs

Update README.md

- Changed README with more compact documentation indicating
  inclusion of iOS support and how the package can be used for both
  production and experimentation.

Removed dart:io usage for any platform failure on apps compiling
with it and trying to use any functions leading to Platform usage
crashes

Further enforcement of using universal_io