[keyserver] moving getting neynar config to function

Summary:
In the next diff, verifying the webhook event will require the webhook secret introduced in D13701. Moving grabbing the neynar config to a function to reduce code duplication

Depends on D13841

Test Plan: Later used in a future diff which successfully grabs the webhook secret

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13755