server: support private key authentication

[conradoplg]

Closes #334

This adds support for private key authentication to the server.

It also changes the frost-client participant and coordinator commands to use it, making username&password not used.

I think we can now remove username&password support and clean up a lot of code. However I thought it would be best to do that in a separate PR which I'll create as a follow up to this.

To test (adjust / run in different terminals where needed)

cargo run -p frost-client -- init -c alice.toml
cargo run -p frost-client -- init -c bob.toml
cargo run -p frost-client -- init -c eve.toml
RUST_LOG=debug cargo run --bin server
# Replace group pubkey and user pubkeys
cargo run -p frost-client -- coordinator -c alice.toml --server-url localhost:2744 --group c0e56fbe3b18eb4536b48a1c50eea7966bead731e203c8eb34af2a3aa43d1e19 -S 5c6024bf25bbd40ac1b4168cc0b5b766c4c546c00accfc2289389a2eabc8b348,58189d9af5a749bf6bf466c97ab5cdf6ed9ce61cec5087ca42bba736cdd2ac55 -m msg.raw
cargo run -p frost-client -- participant -c alice.toml --server-url localhost:2744 --group c0e56fbe3b18eb4536b48a1c50eea7966bead731e203c8eb34af2a3aa43d1e19
cargo run -p frost-client -- participant -c bob.toml --server-url localhost:2744 --group c0e56fbe3b18eb4536b48a1c50eea7966bead731e203c8eb34af2a3aa43d1e19

[conradoplg]

Also use PublicKey from below

[conradoplg]

Done in d3404b6

[natalieesk]

Looks good