Signature Packet format fixes

[vanitasvitae]

This PR does:

• Fix PGPSignature.join() to retain the signature packet length encoding format of the base signature. Previously, the merged result signature object would have its packet length format forced to LEGACY, no matter what the input signatures were encoded with. This would result in mismatches when doing comparisons between encodings in PGPainless' tests.
• Fix PGPSignatureSubpacketGenerator.addEmbeddedSignature() to retrieve the encoding of the embedded signature in legacy format. Otherwise, signatures using the new packet length encoding format, which might have a different header length would result in broken embedded sigs, since the wrong number of header bytes would be cut stripped off.
• Set the packet length encoding format of salted (= recently specified) signatures to the new packet format. This is done in an effort to begin phasing out producing the legacy packet length format.

Note: These changes are currently still part of #1911