Map out asset input and output data structures for CLI

[Isaiah-Turner]

What

We need to map out what information we get about assets from the new ingestion system. We also need to know what the output to BigQuery should look like. Part of #19.

Why

This issue helps us know what asset information is easily accessible from the ingestion system and what information may have to be reconstructed or omitted.

[Isaiah-Turner]

According to the documents on issuing assets, an asset is issued when an account makes a payment operation using that asset. We can use a LedgerTransactionReader and look for operations of OperationTypePayment. When we find an operation of that type, we can save it to be exported. We should only export assets that we haven't seen before. We can use the following code to get asset info:

assetType, code, issuer := new(string), new(string), new(string)
err := op.Body.PaymentOp.Asset.Extract(assetType, code, issuer)

In order to reconstruct the asset table in BigQuery we need the following fields:

type: string - get from assetType
issuer: string - get from code
code: string - get from issuer