Support for signing secureboot artifacts using Hashicorp Vault / OpenBao

[Maks1mS]

Feature Request

Description

Imager currently supports AWS KMS and Azure Key Vault as signing providers for SecureBoot artifacts, but there is no support for HashiCorp Vault (or OpenBao).

[stereobutter]

An alternative to adding a provider for every KMS under the sun would be to add a generic KMS provider that allows the user to hook in some glue code e.g. via an exec based interface or a rest api. We have some internal tooling for code signing for example that we might be able to use that way.

[Jaakkonen]

Created #13280 to allow external signing extensions