Unnecessary custom base64 implementation

Go has [RawStdEncoding](https://pkg.go.dev/encoding/base64#pkg-variables) which makes this unnecessary:

https://github.com/kataras/jwt/blob/1639fcff96f82f7ff118fcff6e1fbd0e01754f2c/token.go#L247

Not trying to nitpick here, rather I think security-wise it's dangerous as a concept to mutate untrusted input data before it's fed to a signature validation algorithm