KMS Plugins for Sigstore

Cosign and private deployments of Fulcio and Rekor can use a KMS-managed key for signing artifacts. We currently have built-in support for AWS, Azure, Google Cloud Platform, and Hashicorp Vault KMSs. This has been a challenge for customers that require alternative or custom KMS solutions. To enable such use-cases, we have implemented a new plugin system for alternate KMS providers. Organizations can independently and privately develop & distribute their plugins without needing downstream updates to libraries to support additional KMS providers as build-time dependencies.