Add support for mapping columns to pubsub message event_timestamp when using flat schemas DDL

Context: https://lists.apache.org/thread.html/bf4c37f21bda194d7f8c40f6e7b9a776262415755cc1658412af3c76%40%3Cdev.beam.apache.org%3E

The syntax should look something like this (proposed by [~alexvanboxel]):
```

CREATE TABLE people (
my_timestamp TIMESTAMP *OPTION(ref="pubsub:event_timestamp)*,
my_id
VARCHAR *OPTION(ref="pubsub:attributes['id_name']")*,
name VARCHAR,
age INTEGER
)
TYPE
'pubsub'
LOCATION 'projects/my-project/topics/my-topic'

```

This jira pertains specifically to the my_timestamp portion of this example.

Imported from Jira [BEAM-9209](https://issues.apache.org/jira/browse/BEAM-9209). Original Jira may contain additional context.
Reported by: bhulette.

The issue specifies a flat-schemas DDL mapping from a Pub/Sub event_timestamp to a timestamp column, but names no source file, test, or entry point. Start by locating Beam SQL DDL handling for Pub/Sub tables and determine where event_timestamp references are resolved; done means the proposed syntax works for the timestamp mapping and is covered by appropriate tests.

Written by the indexing model from the issue text.