[serde-generate] Add support for nested byte vectors

[tzakian]

Summary

This PR adds support for serialization/deserialization of vector<vector<u8>> to the serde-generate runtimes for C#, Go, Java, and Python.

Test Plan

Added additional tests to make sure serialization and deserialization works.

[ma2bd]

@tzakian Why do we need this?

[tzakian]

@MatBD In the process of adding support for vector<vector<u8>> arguments to the transaction-builder generator I ran into issues where the underlying runtime for the generated code in C#, Go, Java, and Python didn't have support for serializing/deserializing vector<vector<u8>> (/Vector<Bytes>) and I couldn't see anywhere where support for that type existed, so this PR is adding in support for that type to the serialization/deserialization runtimes.

[tzakian]

After discussion I'm going to close this PR out in favor of the newtype approach.