Multiple external tables

Figured out there might be multiple external tables passed with a query. I would like to add support for that feature to this client.

My envision is to add
```
type Query struct {
// ...

// keep for backward compatibility
ExternalData []proto.InputColumn
ExternalTable string

// more tables
ExternalTables []ExternalTable
}

type ExternalTable struct {
Name string
Data []proto.InputColumn
}
```

Encoding would be:
* If `ExternalData` is not empty, encode it, just like it's now.
* If `ExternalTables` is not empty, encode each additional external table.

So it can be used the old way setting `ExternalData`, can be used with both set `ExternalData`+`ExternalTables`, can be used with only `ExternalTables`.

How does it look like?