Rust: serde json encode VecM<u8> as base64

[leighmcculloch]

What

Change the serde json encoding of VecM<u8> as base64 in the Rust generated code.

Why

It is common to transmit binary as base64 in JSON and most JSON tooling has good support for dealing with it in this format. e.g. jq.

Follow up to #112 #117

[leighmcculloch]

In the PRs I'm implementing this I've gone with hex instead of base64. While I prefer base64, and in most of our tools we appear to favor base64 for variable length binary data, in Soroban we use variable length binary fields frequently for conceptually fixed length fields. This means more often than not we have things like hashes or IDs in the vararray fields. I think we'll benefit by using hex since we often look at these types of data items in hex. It's also not much of a downside since it is just as trivial to encode and decode hex as it is base64 in most places.