[services] redact server keypair in config struct

Summary:
We should not reveal the keypair in our logs. I've manually implemented the Debug trait to redact the server_keypair field.

Resolves https://linear.app/comm/issue/ENG-1298/redact-sensitive-fields-in-types-that-implement-debug-trait

Test Plan:
Printed the Config struct and confirmed the keypair field was redacted

Config { server_keypair: "redacted" }

Reviewers: palys-swm, karol-bisztyga, jimpo

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4387