Support for coded streams

C++ and Java implementations of protocol buffers have support for [writing and reading multiple messages](https://protobuf.dev/programming-guides/techniques/#streaming) via `CodedOutputStream` and `CodedInputStream` . This is also space-efficient because the size of each message can be represented as a `varint`.

I think having something similar to `CodedOutputStream` and `CodedInputStream` in `betterproto` would be useful. One use case would be when a Python tool uses `betterproto` to emit a binary stream of messages using this and then a C++/Java tool can use `CodedInputStream` to read the stream.

Related Java documentation:
* [CodedInputStream](https://protobuf.dev/reference/java/api-docs/com/google/protobuf/CodedInputStream.html)
* [CodedOutputStream](https://protobuf.dev/reference/java/api-docs/com/google/protobuf/CodedOutputStream)