add frost-client {coordinator,participant}

[conradoplg]

Based on #309

These are basically the same as before, but only supporting the server and reading keys from the config file.

Test is still manual. Generate keys as in #309 and then, e.g.

# Run this and get the group ID (public key)
cargo run -p frost-client -- groups -c alice.toml
# Run these in separate terminals, replace values accordingly
cargo run -p frost-client -- coordinator -c alice.toml --server-url localhost:2744 --group 7c9314b669a334af107f02b7c4d5dfee4e8cafaa4f14907c78d1dfdb17568927 -S alice,bob -m msg.raw
cargo run -p frost-client -- participant -c alice.toml --server-url localhost:2744 --group 7c9314b669a334af107f02b7c4d5dfee4e8cafaa4f14907c78d1dfdb17568927
cargo run -p frost-client -- participant -c bob.toml --server-url localhost:2744 --group 7c9314b669a334af107f02b7c4d5dfee4e8cafaa4f14907c78d1dfdb17568927

Still no encryption&authentication per se, that's #179

[natalieesk]

Looks good