start package cocoon_server for shared server-side cocoon code

[yjbanov]

app_dart and auto_submit provide essentially the same service from two different entrypoints. Right now it's annoying to have to maintain copies of various services like Bigquery, API auth clients, datastore, firestore, graphql, REST API, etc. Every time we decide to move a piece of functionality between app_dart and auto_submit it involves unnecessary code copying and movement. For example, notice how #4043 had to duplicate deleteBranch in app_dart's bespoke github_service.dart.

This PR introduces a local package cocoon_server depended on by both app_dart and auto_submit. This PR only moves bigquery.dart and access_client_provider.dart. Future PRs can move more shared code into cocoon_server. Eventually, app_dart and auto_submit could become thin shells over cocoon_server that simply adapt it to their respective runtime environments.