[keyserver] add signer_uuid and neynar webhook secret

Summary: This adds optional signerUuid and neynarWebhookSecret as options to our neynar config

Test Plan: Accessed values in a later diff. flow check

Reviewers: varun, ashoat, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13701