createPreKeyBundle helper is needed

[microshine]

Example

protected async randomBundle() {
    const preKeyBundle = new PreKeyBundleProtocol();
    await preKeyBundle.identity.fill(this.identity);
    const preKeyId = getRandomInt(1, this.identity.signedPreKeys.length).toString();
    preKeyBundle.preKeySigned.key = this.identity.preKeys.load(preKeyId.toString()).key.publicKey;
    await preKeyBundle.preKeySigned.sign(this.identity.signingKey.privateKey);
    return preKeyBundle.exportProto();
}