crypto: remove default encoding from sign/verify
getDefaultEncoding() always returns 'buffer' in Node.js 20. It requires some careful justification but the default encoding can be eliminated from sig.js entirely.
In Sign.prototype.update, we can safely remove the conditional assignment of getDefaultEncoding() to encoding. This is because SignUpdate() in crypto_sig.cc internally calls node::crypto::Decode, which returns UTF8 for falsy encoding values. In other words, with the conditional assignment, StringBytes::Write() ultimately receives the encoding BUFFER, and without the conditional assignment, it receives the encoding UTF8. However, StringBytes::Write() treats both encodings identically, so there is no need to deviate from the internal default encoding UTF8.
In Sign.prototype.sign, we can also safely remove the conditional assignment of getDefaultEncoding() to encoding. Whether encoding is falsy or 'buffer' makes no difference.
In Verify.prototype.verify, we can also safely remove the conditional assignment of getDefaultEncoding() to sigEncoding. This is because the function passes the sigEncoding to getArrayBufferOrView(), which passes it to Buffer.from(). If sigEncoding is 'buffer', getArrayBufferOrView() instead passes 'utf8' to Buffer.from(). Because the default encoding of Buffer.from() is 'utf8', passing a falsy encoding to getArrayBufferOrView() instead of 'buffer' results in the same behavior.
This partially addresses: