feat: adds cert-utility templates and documentation.

[ianhundere]

closes #886, closes sigstore/fulcio#1930

Summary

currently, there is no standard method for creating cert chains for fulcio or tsa. the community has used an assortment of open source scripts/tools, but i thought it would be nice to have a small cloud agnostic go app to create/sign (via awskms, gcpkms, or azurekms) certificates. the smallstep crypto library is fairly comprehensive in its kms/cert capabilities.

@haydentherapper / @bobcallaway gave the go ahead in proceeding w/ this work.

Release Note

• Adds certificate utility to create and sign certificates via AWS KMS, Google Cloud KMS, or Azure Key Vault.

Documentation

added docs to ./docs folder and updated README.md to point to docs.

[ianhundere]

i think this is ready for 👀 now. just a couple of notes.

1. the following use-cases are now covered:

• root ca -> leaf
• root ca -> intermediate ca -> leaf

2. the following kms providers are working:

• awskms
• azurekms
• gcpkms

3. hashivault was added, but has not been tested.

i think that about covers it, i have some basic readme/documentation above as well.

cc @haydentherapper

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.73%. Comparing base (6fd19b0) to head (a96f1ed).
Report is 282 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #889      +/-   ##
==========================================
- Coverage   52.85%   44.73%   -8.12%     
==========================================
  Files          20       55      +35     
  Lines        1209     3657    +2448     
==========================================
+ Hits          639     1636     +997     
- Misses        509     1881    +1372     
- Partials       61      140      +79     

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[ianhundere]

this is ready for 👀

cc @haydentherapper

[haydentherapper]

Thanks, just a few minor comments!

[ianhundere]

Thanks, just a few minor comments!

done and done / thanks for the 👀.