feat: convert buffers to base64 for json contentType

[nilslice]

This adds conversion functions for all generated types which handles []const u8 data and encodes/decodes base64 before passing back and forth across the wasm boundary.

Of note: all schema types now have __decodeBase64Fields and __encodeBase64Fields public methods. I couldn't get private methods to work, since we import these types around the generated code... if anyone has ideas of how to improve this, or make it more clear via naming that these shouldn't really be called, let me know!

I thought of XXX__decodeBase64Fields or something along those lines, which I think protobuf does in Go for example... and would be open to it. But it's a little jarring IMO to see these in your autocomplete etc. Maybe its more clear?

CI likely fails until we merge & release dylibso/xtp-bindgen-test#10