add frost-client trusted-dealer

[conradoplg]

Based on #308

Closes #306

Just manual testing for now:

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
cargo run -p frost-client -- trusted-dealer --names Alice,Bob,Eve -c alice.toml -c bob.toml -c eve.toml -C ed25519

This PR:

• Refactors the trusted-dealer crate so its main logic can be imported
  • Feedback is welcome on the API (should we keep Config struct or directly add the params?)
• Adds a trusted-dealer subcommand to frost-client which uses the trusted dealer logic, but write the key packages to config files.
  • Currently this is mainly useful for tests. For real world usage we might want to add support to frost-server, or/and to exporting the SecretShare as a blob and importing it with e.g. trusted-dealer-import. But we can do that later.

[natalieesk]

This looks good