Embedding Perfetto in another project

*** note
This doc is WIP, stay tuned

***

This doc should:
- Contain tech details of the Producer(Endpoint), Consumer(Endpoint) and Service
interfaces.
- Explain how they are supposed to be wired up together, with or without
using an IPC transport.
- Explain the basic embedder requirements (e.g. TaskRunner)
- Point out the relevant GN targets:
//src/tracing, //src/tracing:ipc, //src/ipc.
- Explain the API surface:
- producer.h
- consumer.h
- service.h
- Explain the ABI surface:
- shared_memory_abi.h
- IPC's wire protocol (if used)
- The input config protos
- The output trace protos

Other resources