fix: improve handling of buffer type in JSON encoded structs

[zshipko]

• Uses base64 for encoding/decoding buffer values in JSON encoded structs, unfortunately extism_convert::Base64 doesn't work here since it doesn't implement serde::{Serialize, Deserialize}
• Updates xtp-bindgen so we can use the built-in camelToSnakeCase and capitalize functions

[nilslice]

This LGTM! I'm just going add a new test to the bindgen suite to check the round-trip and then re-run from here.