add bigquery connection with examples

[modular-magician]

hashicorp/terraform-provider-google#5530

notes:

1. it's unclear on if this is ga or not to me, the api url says beta, so i went with beta.
2. I had to declare google-beta as the provider for everything in the examples, or the terraform go modules wouldn't build, what's the desired way to do this?
3. name vs id was not clear to me, i mapped the properties directly from the api, but this leads to the oddly reading:
   instance_id = google_sql_database_instance.instance.connection_name
   per the docs, connection_id is optional (left blank a default is generated for you), which seems different from other products
4. "DATABASE_TYPE_UNSPECIFIED" when set maps to an undefined value on response (the type property is not present in the response) - I couldn't find any code to cargo-cult here, suggestions?

`google_bigquery_connection`

Derived from GoogleCloudPlatform/magic-modules#3424