Native Payments SDK implementation

[cyucelen]

I implemented Klarna Native Payments SDK for flutter, there might be lacks or code style inconsistencies in implementation, I'm up for providing fixes to this PR if you give some feedbacks!

You can test the application using example application I provided under examples/payments folder.

Types of changes
What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist
Go over all the following points, and put an x in all the boxes that apply. If you're unsure about any of these, don't hesitate to ask.

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[CLAassistant]

All committers have signed the CLA.

[ElDuderini]

@NMGuner Should have some eyes on this to add the extra support the Klarna plugin needs to be on par with the native solution.

[BunnyBuddy]

If I want to test this how can I do it? I tried using this PR but its using pretty old things i.e
saw flutter test and mockito dependency issue and its using android embedding v1 which is deprecated.
I tried running the payments example

[BunnyBuddy]

getting this error in your payments example even if I use a working client token

I implemented Klarna Native Payments SDK for flutter, there might be lacks or code style inconsistencies in implementation, I'm up for providing fixes to this PR if you give some feedbacks!

You can test the application using example application I provided under examples/payments folder.

Types of changes What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist Go over all the following points, and put an x in all the boxes that apply. If you're unsure about any of these, don't hesitate to ask.

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Can you please guide me as how can I resolve this?