[identity] load OPAQUE server setup from environment variable

Summary:
The Identity service now expects the OPAQUE_SERVER_SETUP environment variable to be set at runtime.

When the keygen command is run, the serialized server setup bytes are now base64 encoded before they're written to the file system. This base64 encoded string must then be made available with the above environment variable when the server is run.

Test Plan:
ran the keygen program, retrieved the base64 encoded string, ran the server program with OPAQUE_SERVER_SETUP=<encoded-string> and the program succeeded.

Also tried supplying an invalid string via env var and the server program crashed, as expected.

Reviewers: jon, bartek!

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8641