Multiple external tables

[nikandfor]

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?