Get the signing state directory for a group (without session).
Path: {registry_dir}/group-state/{group_id.hex()}/signing
{registry_dir}/group-state/{group_id.hex()}/signing
Port of signing_state_dir_for_group() from cmd/sign/common.rs.
signing_state_dir_for_group()
Get the signing state directory for a group (without session).
Path:
{registry_dir}/group-state/{group_id.hex()}/signingPort of
signing_state_dir_for_group()from cmd/sign/common.rs.