Warn about usage saltLength: 0 for RSA PSS sign/verify

[olegbespalov]

What?

While implementing support of RSA #85 we've found that standard Golang's SDK doesn't provide support of the saltLength: 0, instead it always tries to generate salt with the maximum length.

This led us for the patch to WebPlatfrom tests:
https://github.com/grafana/xk6-webcrypto/blob/main/webcrypto/tests/wpt-patches/WebCryptoAPI__sign_verify__rsa.js.patch

Since some use cases could actually try to use the lengths of salt 0 we need to issue a log warning user that the actual behavior of our implementation is different.

Why?

It improves UX and makes implementation predictable.