[Task]: Add support for custom data for structured logging

### What needs to happen?

This issue tracks the implementation of https://s.apache.org/beam-structured-logging.
The feature will allow adding custom data fields to log entries.

High-level steps:
1. Adding a new field in `LogEntry` (beam_fn_api.proto)
2. Adding support for structured logging in Java, Python, and Go SDKs.
3. Adding support for new field in Dataflow, possibly other runners (contributions welcome).

### Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

### Issue Components

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