Add exchange transaction method

[johnmaia]

This PR adds the exchange transaction method.

That accepts the following parameters:

• baseAmount (required)
• baseAssetCode (required)
• code
• commit
• customData
• description
• destinationAddress
• externalId
• externalReference
• originAddress
• quoteAmount (if given, then rate should not be included)
• quoteAssetCode (required)
• rate (if given, then quoteAmount should not be included)
• walletId (required)

If the quoteAmount and rate are not given on the creation, the exchange will accept those values when approving the transaction. In case the code is set to internal and the quoteAmount and rate are not given on the creation, the exchange will use the domain's rate for the given currency pair (baseAssetCode/quoteAssetCode) if it exists.

If the quoteAmount or rate are given or retrieve from the domain's rate (in case of the internal exchange) the transaction will be automatically approved. If the commit is also sent as true then the transaction will be approved and then automatically confirmed.