[identity] add optional keyserver public key to config

Summary:
in production, we will hardcode the content ed25519 public key for ashoat's keyserver in the identity service config.

this will let us verify signed messages from ashoat's keyserver.

Test Plan: successfully printed the public key in main when i set the environment variable

Reviewers: jon, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8231