Hashicorp Vault Templating

**Describe the problem/challenge you have**
I would like to use Vault to provide my secrets for my YTT templates. To achieve this, one can encrypt the secrets YAML with the Vault Transit engine and store in their repository.

**Describe the solution you'd like**
A templater in kapp controller that supports Hashicorp Vault's transit engine to decrypt files similar to the SOPS templater. This way I can decrypt secrets, template with ytt, and deploy with kapp.

**Additional Details**
It is also possible to use Vault to inject secrets, you just would not be able to use those secrets with ytt. See [here](https://www.hashicorp.com/blog/injecting-vault-secrets-into-kubernetes-pods-via-a-sidecar) and [examples](https://www.vaultproject.io/docs/platform/k8s/injector/examples)