Include schema when publishing results to BigQuery

Using `pkb.py --bigquery_table [dataset.table]` isn't intuitive. If the referenced table does't exist, `bq load` fails, and it isn't apparent what the table schema should be, or how to create it. After some digging, I found an [example schema](https://github.com/GoogleCloudPlatform/PerfKitExplorer/blob/master/data/samples_mart/results_table_schema.json) in the [PerfKitExplorer repo](https://github.com/GoogleCloudPlatform/PerfKitExplorer). Use of `--bigquery_table` would be simpler if a default schema was included in this repo, and referenced using the `bq load ` argument.