Calculate fee based on tx size and on-chain tx fee parameters

[webmaster128]

We are planning to charge per byte of signed transaction size: iov-one/weave#1091

In order to calculate the fee, we need to know the final transaction size including aignatures before the the transaction is signed.

Do no this, we can do bnsCodec.bytesToPost(withDummySignatures), where withDummySignatures is the unsigned transaction plus the expected number of signatures. A dummy signature can be constructed as

const dummy: FullSignature = {
  nonce: 66 as Nonce,
  pubkey: {
    algo: Algorithm.Ed25519,
    // ed25519 pubkey has 32 bytes https://blog.mozilla.org/warner/2011/11/29/ed25519-keys/
    data: new Uint8Array(32) as PubkeyBytes,
  },
  // ed25519 signature has 64 bytes https://blog.mozilla.org/warner/2011/11/29/ed25519-keys/
  signature: new Uint8Array(64) as SignatureBytes,
};

The fee calculation constants FREE_CONTINGENT and MULTIPLIER can be fixed in the first step but should be queried from the chain later on.

• Add helper functions for estimating tx size as above
• Add function for getting tx fee parameters from chain
• Update BnsConnection.getFeeQuote to use these functions