Add Python PubSub pipeline for processing gcloud logs

[Amar3tto]

Pipeline added: Google Cloud log processing with Apache Beam and Dataflow
IO: PubSub IO
SDK Language: Python

Run instruction:

• Create PubSub topics: input, output and DLQ.
• Go to Log Router and route logs to the input PubSub topic (e.g. only logs for BigQuery).
• run python gcloud_logs_filter_with_dlq.py --runner="DataflowRunner" --project="your-project" --region="your-region" --input_topic="your-input-topic" --output_topic="your-output-topic" --dlq_topic="your-dlq-topic".
• Perform some actions (e.g. create/delete BigQuery table) so the logs will be routed to the input topic.
• Check output PubSub topic.

[olehborysevych]

LGTM except minor comment

[olehborysevych]

WE will also need to provide a better PR description before sending for external review.

[johnjcasey]

LGTM