signing: change signbytes

[tac0turtle]

Description

have SignBytes accept a proto type

Closes: #XXX

[auto-comment]

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[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (marko/canonical@58c37a6). Click here to learn what that means.
The diff coverage is n/a.

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##             marko/canonical    #4852   +/-   ##
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  Hits                       ?     8869           
  Misses                     ?     4848           
  Partials                   ?      789           

[tac0turtle]

going to merge this into the privval proto migration so it can be reviewed at once