Update PayPal integration and make it agnostic

[liveHarshit]

Describe the bug
Currently, PayPal is configured that it is too frontend/ember data-centric, and not agnostic which makes it difficult for Android apps to do stuff, as there is no standard JSONAPI library and we have to do filtering and resource querying by hand.
Docs - https://developer.paypal.com/docs/archive/checkout/how-to/server-integration

[kushthedude]

@liveHarshit Are you interested working on it or should i try ?

[liveHarshit]

@liveHarshit Are you interested working on it or should i try ?

You can try, I'm not familiar with the server implementation.

[mrsaicharan1]

@liveHarshit Please add AliPay to the title too as soon as #5960 is merged. Would like to fix this as soon as possible.

[liveHarshit]

@mrsaicharan1 Please add the implementation to the server docs also, so I can follow the document instructions.

[liveHarshit]

@mrsaicharan1 @kushthedude What is the status?

[mrsaicharan1]

@mrsaicharan1 @kushthedude What is the status?

Will start working on this as soon as the intial workflow for alipay gateway is done! Here's the status of the PR fossasia/open-event-frontend#3047. As soon as this is merged, I'll get onto implementing server independent