I believe proto-plus / protobuf / grpc packages were only needed for google.cloud.bigquery_v2. These modules have been deprecated and disconnected from the google.cloud.bigquery modules in version 3.x. I believe this means we can move those dependencies to "extras" (a.k.a. optional dependencies) and make our package lighter-weight by default.

Follow-up to https://github.com/googleapis/python-bigquery/pull/1282/files#discussion_r928022130 and https://github.com/googleapis/python-bigquery/pull/1282/files#discussion_r928022317