Update khalti SDK to make use of updated APIs

[Biplab-Dutta]

This PR makes changes in how merchants can use Khalti SDK to make payments.

• Previously, the SDK provided most of the UI elements that should have been incorporated in merchant's app. This change brings a WebView implementation of Khalti Payment Portal for seamless adaptation and payment making process.
• The SDK removes all the deprecated backend APIs with the new APIs.
• Removal of khalti package. Now there are only khalti_core and khalti_flutter packages in this repo.
• Version bump in khalti_flutter (v3.1.0) and khalti_core (v1.2.0) library.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 41.30435% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 55.07%. Comparing base (c925f37) to head (5e495c6).

Files	Patch %	Lines
...ages/khalti_core/lib/src/config/khalti_config.dart	26.66%	11 Missing ⚠️
...ages/khalti_core/lib/src/model/payment_result.dart	0.00%	8 Missing ⚠️
...src/model/payment_verification_response_model.dart	52.94%	8 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           master      #37       +/-   ##
===========================================
- Coverage   95.06%   55.07%   -40.00%     
===========================================
  Files          12        8        -4     
  Lines         304      138      -166     
===========================================
- Hits          289       76      -213     
- Misses         15       62       +47     

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[ishowrunes]

@Biplab-Dutta What are the changes introduced in this PR?

[Biplab-Dutta]

@Biplab-Dutta What are the changes introduced in this PR?

@Ishwor-Shrestha Updated description

[acpmasquerade]

Dart Package for new version

• khalti_checkout

Flutter package for new version

• khalti_checkout_flutter

Project namespace to be renamed as - "Khalti Checkout"

• and first version to be pushed as
  1.0.0-preview-release

[Biplab-Dutta]

Dart Package for new version

• khalti_checkout

Flutter package for new version

• khalti_checkout_flutter

Project namespace to be renamed as - "Khalti Checkout"

• and first version to be pushed as
  1.0.0-preview-release

The changes have been made. The release on pub.dev remains. @acpmasquerade