fdo-manufacturing-client plain-di does work #477

As described in https://fedora-iot.github.io/fido-device-onboard-rs/specs/diun.html,
manufacturing-server needs to get public key prior to the start of the protocol.

Currently manufacturing-server tries to load the public key here:

https://github.com/fedora-iot/fido-device-onboard-rs/blob/69fec227de04d98c51a5b5d8eb218bdc264f0a3f/manufacturing-server/src/handlers/di.rs#L77-L84

but there are several problems for it.

1. The file path is {public_key_store}/{msg_info}, which msg_info is not a pub key file name. It should allow uses to set the path like public_key_path via configfile.
2. It calls store.load_data() to load the public key but load_data() deserialize the public key. It is not practical that users need to set the public key with the format change.

Also, there is no integration test to reveal fdo-manufacturing-client plain-di does not work.