BigQuery: accept a string in addition to a [Dataset|Table]Reference in the [Dataset|Table] constructors #7477

This is a follow-up to #6055

I missed the Dataset and Table constructors when making anywhere that takes a [Dataset|Table]Reference also accept a string. In these constructors, there isn't a project ID available, so the string would have to be fully-qualified (that is, it needs to contain a project ID).