Add asn1 signature encoding to signer and verifier

[code28]

Unfortunately, we're using a library (for another language), which doesn't support concatenating r and s for the signature, so we needed a way to use a common encoding. Since the "original" encoding when used in iOS (or macOS) is ASN1, this data is already available inside the ECSignature.
This change isn't breaking, since without changing anything, the old encoding is used.

I know, this isn't quite correct for the JWS standard, but maybe there are other people running into this problem. What do you think about this option?

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