[serde generate] java runtime: improve Bytes class

[ma2bd]

Summary

• Make the default constructor non-copying (does not prevent sharing but faster)
• Create convenient safe APIs Bytes.empty() and Bytes.valueOf(content)

Test Plan

CI

[thefallentree]

can we consider use guava instead ?

https://guava.dev/releases/27.1-jre/api/docs/index.html?com/google/common/primitives/Bytes.html

in general this is the defacto standard for java these days

[ma2bd]

@thefallentree Good question. So far, we've managed to keep the LCS runtimes mostly dependency-free (the only exception I can remember is numpy in python). It seems a bit vexing to pull guava just for Bytes. (FWIW protobuf does not)