[Feature Request]: C++ SDK

### What would you like to happen?

It would be great to have C++ SDK in order to create very high-performant yet safe pipelines.

My personal interest is in writing Beam pipelines for recommender systems, however given the [increasing usage of C++](https://www.tiobe.com/tiobe-index/) and wide applicability of Beam, I was wondering if this might be well received by the developer community.

### Issue Priority

Priority: 3 (nice-to-have improvement)

### Issue Components

- [ ] Component: Python SDK
- [ ] Component: Java SDK
- [ ] 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