bigquery: breaking changes for v2

[jba]

Consider these breaking changes for v2 of the client:

• Rename FullyQualifiedName to FullyQualifiedID.
• Instead of LoaderFrom, a LoadFrom method that takes a LoadConfig. Similarly for CopierFrom and ExtractorTo.
• Rethink how external tables and data are specified. GCSReference should not have to implement ExternalData for backwards compatibility.
• Rethink load sources. Now LoadSource is implemented by ReaderSource and GCSReference. Is there a clearer design?
• Rename the QueryConfig.Q field.
• Rethink the loader/saver design, especially the names ValueSaver and ValuesSaver.

[jba]

• Remove the Value type and just use interface{}. This would avoid unnecessary conversions. See also bigquery: simplify inserting rows from JSON #845.

[kurtmaile]

Agreed would be nice.

[jeanbza]

Tracking in internal bug 135054263.

[derekperkins]

The Inserter interface leaves a lot to be desired. I'd much prefer to have an option to pass in json that I've marshaled rather than having to jump through hoops to get map[string]bigquery.Value. We have some complex structs where we double marshal to get data into that format.

func (o *ComplexBQObject) Save() (map[string]bigquery.Value, string, error) {
	data, err := json.Marshal(o)
	if err != nil {
		return nil, "", e.Wrap(err)
	}
	toSave := map[string]bigquery.Value{}
	if err = json.Unmarshal(data, &toSave); err != nil {
		return nil, "", e.Wrap(err)
	}
	return toSave, "", nil
}