Because:
* Nx gives us the ability to break things up into smaller libraries with
individual responsibilities.
* Paypal logic can exist in a standalone library for reusability in SP3
This commit:
* Moves PayPalClient and associated utils/typings to a library named
`subs/paypal`. It can be imported via `@fxa/payments/paypal` in any
TypeScript code.
Closes FXA-7610