signature file extension generated by blob sign

[Two-Hearts]

This issue is linked to: notaryproject/notation#974

According to OpenSSF Scorecard signing check criteria, only these signature file formats *.asc (pgp), *.sig, *.sign, *.sigstore, *.intoto.jsonl could be detected by OpenSSF Scorecard check tool.

However, current notary project spec has .sig.jws and .sig.cose file format.
We could switch the order of envelope format and the .sig extension. Therefore, the suggested format are .jws.sig and .cose.sig.