Convenient Coin APIs

[AlicanC]

feat!: add TransactionRequest classes

• Replaces ScriptTransactionRequest/CreateTransactionRequest types with classes that have methods like addCoins, addContract, etc. to make things easier.

feat: add wallet coin APIs

• Adds methods: getCoinsToSpend, getCoins, getBalances and transfer
• Adds fund(tx) method which calculates the cost of the tx and adds coins from the wallet to the tx to cover it.

feat: add CoinQuantity

• Adds CoinQuantity which is a combination of an amount and an AssetId. This replaces SpendQueryElement, and instead of just { amount, assetId } it also accepts [amount, assetId], allowing simpler APIs.

feat: add interfaces package

• Adds @fuel-ts/interfaces, which is similar to fuel-core-interfaces. Currently contains AbstractWallet and AbstractContract. This allows working with Wallets and Contracts without importing their @fuel-ts/xyz packages and prevents circular deps.

---

A transfer (single-source/multi-destination/multi-asset) after these changes:

const coins = await sender.getCoinsToSpend([
  [amount.mul(2), assetIdA],
  [amount.mul(2), assetIdB],
]);

const request = new ScriptTransactionRequest({ gasLimit: 1000000 });
request.addCoins(coins);
request.addCoinOutputs(receiverA, [
  [amount, assetIdA],
  [amount, assetIdB],
]);
request.addCoinOutputs(receiverB, [
  [amount, assetIdA],
  [amount, assetIdB],
]);

const response = await provider.sendTransaction(request);

Before:

const coins = await provider.getCoinsToSpend(sender, [
      { assetId: assetIdA, amount: amount.mul(2) },
      { assetId: assetIdB, amount: amount.mul(2) },
    ]);

await provider.sendTransaction({
  type: TransactionType.Script,
  gasPrice: BigNumber.from(0),
  gasLimit: BigNumber.from(1000000),
  bytePrice: BigNumber.from(0),
  script: returnZeroScript.bytes,
  inputs: coins.map((coin) => ({
    type: InputType.Coin,
    ...coin,
    witnessIndex: 0,
  })),
  outputs: [
    {
      type: OutputType.Coin,
      to: receiverA.address,
      assetId: assetIdA,
      amount,
    },
    {
      type: OutputType.Coin,
      to: receiverA.address,
      assetId: assetIdB,
      amount,
    },
    {
      type: OutputType.Coin,
      to: receiverB.address,
      assetId: assetIdA,
      amount,
    },
    {
      type: OutputType.Coin,
      to: receiverB.address,
      assetId: assetIdB,
      amount,
    },
    { type: OutputType.Change, assetId: assetIdA, to: sender },
    { type: OutputType.Change, assetId: assetIdB, to: sender },
  ],
  witnesses: ['0x'],
});

[AlicanC]

Support submitting multiple transactions at once #194 was going to be resolved in this PR, but I've found a better way to do it which is blocked by a fuel-core issue so I left it to not block this.

[digorithm]

LGTM. Great work on this, the API looks pretty neat and clean!