Move the APIs out to a dedicated repo

[myidpt]

It would be worthwhile to move the APIs in https://github.com/thalescpl-io/k8s-kms-plugin/tree/master/apis to a dedicated repo, such as https://github.com/thalescpl-io/k8s-kms-plugin-apis. This will greatly remove the dependencies.

For example, the istio/api go.mod file is very short: https://github.com/istio/api/blob/master/go.mod

Currently consumers of k8s-kms-plugin have to import all the dependencies of k8s-kms-plugin, making it hard to manage the go dependency versions. So far we have seen the licensing issues and the golang.org/x/sys version compatibility issue #26.

[hoskeri]

SGTM, I want to make sure the build of the plugin binary itself continues to stay hermetic.

So, after the APIs are moved to the other repo, there should be a vendor'd copy of the apis repo at build time.