BigQuery Storage: Convert DATETIME columns to datetime.datetime objects when using Avro wire format

[tswast]

Limitation currently documented here:

https://github.com/googleapis/google-cloud-python/blob/6a06a5f3b99ccd2f7f5f003d2da08b501bdf55d2/bigquery_storage/google/cloud/bigquery_storage_v1beta1/reader.py#L152-L154

We can check for DATETIME columns based on the Avro schema. There's a logical type annotation for the field.

I had hoped that perhaps this logical type could be part of the Avro spec, but the community is going in a different direction: https://issues.apache.org/jira/browse/AVRO-2328

[plamut]

Also, when done, a system test needs to be updated - https://github.com/googleapis/google-cloud-python/pull/8992/files#diff-4af9660141e2ded70b95a2dfb41eebbeR386

[chalmerlowe]

Due to current workloads and prioritizations, closing as "Will not fix".