Update ffiPaymentRedirect to use compute

[atavism]

• Update ffiPaymentRedirect to use compute
• Add PaymentRedirect API response type
• Update email exists check to return string instead of pointer

This also fixes an issue on main with desktop where the following error is shown after opening the payment redirect page

[jigar-f]

I am good with this @atavism, Merging...

[atavism]

Thanks for merging and the review, @jigar-f!