ReadFromBigQuery doesn't support DATETIME type

ReadFromBigQuery with Avro export by documentation should produce a python native object i.e. datetime.datetime. 

\> the BigQuery types for DATE, DATETIME, TIME, and TIMESTAMP will be exported as strings. This behavior is consistent with BigQuerySource. When using Avro exports, these fields will be exported as native Python types (datetime.date, datetime.datetime, datetime.datetime, and datetime.datetime respectively)

However, in practice this doesn't happen as BigQuery doesn't have a type that maps to Avro logical type.

\> *Note:* There is no logical type that directly corresponds to `DATETIME`, and BigQuery doesn't support any direct conversion from an Avro type into a `DATETIME` field.

I've also checked manually the AVRO export file that get exported to GCS. The logical type in Avro schema for BQ is `datetime`.

{"name":"datetime_col","type":["null",\{"type":"string","logicalType":"datetime"}]}

Avro spec doesn't support `datetime` logical type[3] nor FastAvro library[4] that is used underneath to read the avro files.

Resources:

[1] [https://beam.apache.org/releases/pydoc/2.29.0/apache_beam.io.gcp.bigquery.html#apache_beam.io.gcp.bigquery.ReadFromBigQuery](https://beam.apache.org/releases/pydoc/2.29.0/apache_beam.io.gcp.bigquery.html#apache_beam.io.gcp.bigquery.ReadFromBigQuery)

[2] [https://cloud.google.com/bigquery/docs/loading-data-cloud-storage-avro#logical_types](https://cloud.google.com/bigquery/docs/loading-data-cloud-storage-avro#logical_types)

[3] https://avro.apache.org/docs/1.8.0/spec.html#Logical****Types|https://avro.apache.org/docs/1.8.0/spec.html#Logical****Types]

[4] https://fastavro.readthedocs.io/en/latest/logical_types.html

Imported from Jira [BEAM-12375](https://issues.apache.org/jira/browse/BEAM-12375). Original Jira may contain additional context.
Reported by: sayatez.