[Feature Request]: Add GCP Firestore IO connector for Python SDK

### What would you like to happen?

Similar to how the [Kafka cross-language transform](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/kafka.py) was built, it would be really helpful if Firestore IO connector was made available in Python SDK by using the Java connector through a external transform.

I tried doing it myself but I wasn't able, so having this connector available in the beam package would help many non-advanced users like me to use Firestore as a sink

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

### Issue Components

- [x] Component: Python SDK
- [ ] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam YAML
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Infrastructure
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner