[identity][docker] Add option to generate opaque keypair

Summary:
I'm aware that this functionality existed before, but was removed in D8580. Now I'm reintroducing it but making opt-in.
For commtest being run as a docker container that depends on identity, the easisest way is to generate the opaque server setup during image build.

Depends on D9507

Test Plan:

• docker compose build identity-server creates unchanged image
• docker compose build --build-arg 'generate_keypair=true' adds additional layer and generates secrets/server_setup.txt

Image layers can be checked using e.g. the dive tool: dive commapp/identity-server.

Reviewers: varun, michal, jon

Reviewed By: varun

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9508