[identity] check that incoming requests contain valid auth token

Summary:

• statically initialize config so that we can access it from request interceptor

• write interceptor function that compares authorization key from request metadata with the config token value

• remove config param from other functions now that the config is static

Test Plan:

• first, ran the server and verified that the process exits if the AUTH_TOKEN variable is not present
• then, set the env var and attempted to send requests to the server without any metadata; they were rejected as expected
• lastly, sent a request with authorization key set in the metadata and received a successful response

Reviewers: jon, bartek, tomek

Reviewed By: jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6824