spike/poc: cert create/kms-sign using smallstep's crypto library

[ianhundere]

Description

as an eng
i want to be able to easily/effectively create/kms-sign certs for fulcio and/or tsa
so that the developer experience for sigstore svcs is much more straightforward and easy to implement

acceptance criteria:

• configure aws access
  • create kms keys
• standup sigstore locally
  • rekor
  • fulcio
  • tsa
• test:
  • sigstore-cert-creator using kms keys
  • use custom CAs/leaves w/:
    • fulcio
    • tsa

helpful links:

• AWS KMS documentation Unclear sigstore/fulcio#1178 (comment)
• https://github.com/smallstep/crypto
• https://github.com/smallstep/cli

[ianhundere]

• configure aws access
  • create kms keys
• standup sigstore
• test:
  • sigstore-cert-creator using kms keys
  • use custom CAs/leaves w/:
    • fulcio
    • tsa