Fix grant initiation response body

[mkurapov]

Currently, we return expiresIn as part of the grant initiation request:

rafiki/packages/auth/src/grant/routes.ts

Line 420 in 74e6f64

expiresIn: accessToken.expiresIn

However, open payments API expects snake case expires_in:

https://github.com/interledger/open-payments/blob/0523804eeb21d9b8cd2eb65e4119738a0512a095/openapi/auth-server.yaml#L64

We do have an issue open in the open-payments repo for converting to pascal case: interledger/open-payments#195 (which would solve this issue altogether), but I'm more concerned that the Open API request/response validation we have set up allows this to pass. Is it because of the fact that access_token is optional in the case where an interactive grant gets generated?

[mkurapov]

CC @wilsonianb

[wilsonianb]

fwiw, I have an expires_in fix queued up: 9315f07
(I might extract it from that changeset since I'll have the backend importing types from open-payments instead of auth)

I wonder if improving the validation is a matter of adding additionalProperties: false to
https://github.com/interledger/open-payments/blob/0523804eeb21d9b8cd2eb65e4119738a0512a095/openapi/auth-server.yaml#L414