feat: search statistics

[chalmerlowe]

Adds a feature to display search statistics in support of the following customer request: https://buganizer.corp.google.com/issues/290679162

Includes several classes to encapsulate the data found here:
https://cloud.google.com/bigquery/docs/reference/rest/v2/Job#searchstatistics

Also includes a function to access the data in the classes.

[shollyman]

Good start on the basic structure. This is an output-only stat so no need for a setter, but we'll need all the unit testing and a similar IT test to java using a query that invokes the search() sql function.

[shollyman]

need to handle the not-present case, e.g. if stats is not None:

[shollyman]

Generally, we want to avoid the library injecting values not present in the API response, so returning the default value here may not be more desirable than empty/none.

[shollyman]

After some more reading, I think the attribute approach is sufficient vs explicit property annotations.

Main concern is about attribute defaults. Typically None is better when users are trying to disambiguate between presence of a value vs the empty value, but happy to talk about this more. Swapping the default impacts typing, unit tests, etc.