Make GroupKey convertible to PublicKey?

[Nashtare]

Failing to convert to ed25519_dalek::PublicKey half the time due to compressed form encoding. We may want to make this possible in the future to allow verification through ed25519_dalek.

Visible in test signing_and_verification_with_ed25519_dalek_2_out_of_3 in tests/frost.rs

[simonsso]

(for readers in the future, the ticket merged was referring to an issue upstream project not this one)