Supporting alternative base64/json encoder/decoder

Would you be open to supporting alternative base64 implementations? The one we've been using is https://github.com/segmentio/asm/tree/main/base64

It's significantly faster than the stdlib encoding, and takes shortcuts that I don't see why wouldn't be acceptable here.

Ideally, if the token parsers were abstracted to accept a Encoding interface, we could pass in our own, with default behaviors to maintain encoding/base64.

In similar vein, similar can be said for JSON codecs. The stdlib is known for having sub-optimal performance here. Swapping encoding/json for something like, fastjson (https://github.com/valyala/fastjson) would be nice.

I'm new to this project and haven't dug in much, so I don't know all the consequences of this or how to gauge it yet, but I know for us internally, we've adopted these codec alternatives for packages we control.