Gofmt.
Export new function ConvertCreateRawTxParams.

The inputs to the createrawtransaction command are JSON strings, however
the NewCreateRawTransactionCmd function takes concrete types.  Thus,
callers which deal with the raw JSON need a method to perform the
validation and conversion.  This logic is the same as what needs to happen
when unmashalling a full JSON-RPC command, so this commit factors that
logic out into a separate function and exports it.