Add signature algorithm to template during certificate creation

[Horiodino]

Summary

Added ToSignatureAlgorithm to support specifying non-default hash algorithms during certificate creation

#1161

Release Note

Documentation

[haydentherapper]

This looks good overall! We need to also invoke it wherever we call x509.CreateCertificate, like in

fulcio/pkg/ca/baseca/baseca.go

Line 60 in 260872d

finalCertBytes, err := x509.CreateCertificate(rand.Reader, cert, certChain[0], publicKey, privateKey)

or the other CA implementations in pkg/ca.

[haydentherapper]

Can you move this to https://github.com/sigstore/fulcio/blob/main/pkg/ca/common.go?

For context, pkg/certmaker was just added to support a new CLI utility, and I'd prefer to add new functions to it that are only used by the utility.

We should still keep the places above where you updated tmpl.SignatureAlgorithm though, that LGTM.

[codecov]

Codecov Report

Attention: Patch coverage is 77.55102% with 11 lines in your changes missing coverage. Please review.

Project coverage is 52.30%. Comparing base (cf238ac) to head (d29472e).
Report is 292 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/certmaker/certmaker.go	77.55%	8 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1929      +/-   ##
==========================================
- Coverage   57.93%   52.30%   -5.63%     
==========================================
  Files          50       73      +23     
  Lines        3119     5695    +2576     
==========================================
+ Hits         1807     2979    +1172     
- Misses       1154     2439    +1285     
- Partials      158      277     +119     

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