Refactor fulcio signer to take in KeyOpts.

[wlynch]

Summary

This commit should not have change in behavior. It changes the fulcio signer
to take in the KeyOpts struct instead of passing individual parameters through,
as well as moves common code dependent on the values to the same place.

The motivation behind this change is to make it easier to add new options without
needing to plumb through another param to the already long list.

To avoid circular dependencies (sign -> fulcio -> sign), KeyOpts was moved to the
options package.

Ticket Link

Part of #1785 (wanted to make this change first to make adding the option easier)

Release Note

NONE

[codecov-commenter]

Codecov Report

Merging #1788 (0d0e5b6) into main (afaf0a3) will decrease coverage by 0.01%.
The diff coverage is 3.44%.

@@            Coverage Diff             @@
##             main    #1788      +/-   ##
==========================================
- Coverage   31.46%   31.45%   -0.02%     
==========================================
  Files         144      144              
  Lines        8893     8896       +3     
==========================================
  Hits         2798     2798              
- Misses       5758     5761       +3     
  Partials      337      337              

Impacted Files	Coverage Δ
cmd/cosign/cli/attest.go	0.00% <0.00%> (ø)
cmd/cosign/cli/policy_init.go	1.40% <0.00%> (ø)
cmd/cosign/cli/sign.go	0.00% <0.00%> (ø)
cmd/cosign/cli/sign/sign.go	14.69% <0.00%> (+0.33%)	⬆️
cmd/cosign/cli/sign/sign_blob.go	0.00% <0.00%> (ø)
cmd/cosign/cli/signblob.go	0.00% <0.00%> (ø)
cmd/cosign/cli/verify.go	0.00% <0.00%> (ø)
cmd/cosign/cli/verify/verify_blob.go	10.42% <0.00%> (ø)
cmd/cosign/cli/fulcio/fulcio.go	22.10% <7.14%> (-2.90%)	⬇️

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