Single transaction API endpoint

[POST] /api/transactions/single

Should build not signed transaction to transfer from the single source to the single destination. If the transaction with the specified operationId has already been built by one of the `[POST] /api/transactions/*` call, it should be ignored and regular response (as in the first request) should be returned.

Body:

```js
{
// Operation ID
“operationId”: “guid”,
// Source address
“fromAddress”: “string”,
// Destination address
“toAddress”: “string”,
// Asset ID to transfer
“assetId”: “string”,
// Amount to transfer. Integer as string, aligned
// to the asset accuracy. Actual value can be
// calculated as
// x = amount / (10 ^ asset.Accuracy)
“amount”: “string”,
// Ignored . No Fiber coin implements this ... yet
“includeFee”: boolean
}
```

Response:

```js
{
// Error code.
// Can be empty.
// Should be non empty if an error that match one of the
// listed code is occured. For other errors use HTTP
// status codes.
// enum values:
// - amountIsTooSmall: amount is too small to execute
// transaction
// - notEnoughBalance: transaction can’t be executed due
// to balance insufficiency on the source address.
"errorCode": "enum",
// The transaction context which will be passed to the
// [POST] /api/sign.
// Should be not empty when result is successful.
“transactionContext”: “string”
}
```