Add a CLI driver for compos_key_service.

This will eventually go away, but it's useful for verifying that the
service is working. But the cert -> RSAPublicKey code will end up
somewhere else eventually - either in C++ or Rust.

Bug: 191763370
Test: Manual - start service, generate key, verify. Inspect files.
Change-Id: I2181cf5331992a4236500545a9fdfd8640b57c1d
3 files changed