PFI terminology and removing jargon

[michaelneale]

https://github.com/TBD54566975/didpay/pull/171

We probably need to stop saying "PFI" jargon and be clearer with what "things" are in context.

[michaelneale]

cc @ethan-tbd @frankhinek:

• Transfer service
• service provider
• payment service/provider
• financial institution.