Include schema when publishing results to BigQuery

[ghost]

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 in the PerfKitExplorer repo. Use of --bigquery_table would be simpler if a default schema was included in this repo, and referenced using the bq load <table_schema> argument.

[voellm]

Good feedback.