[Identity] Support keygen and server setup for opaque 2.0

Summary:
Opaque 2.0 requires different setup for it to run.
This adds support generating the secrets and adding the
ServerSetup object to the configuration.

Test Plan:

cd services/identity
cargo run -- keygen
cargo run -- keygen # should skip writting files
cargo run -- server

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7299