STAC-24174: add stackpack validate command

[deontaljaard]

# Validate using API
sts stackpack validate --name my-stackpack

# Validate using Docker with a directory
sts stackpack validate --image quay.io/stackstate/stackstate-server:latest --stackpack-directory ./my-stackpack

# Validate using Docker with a file
sts stackpack validate --image quay.io/stackstate/stackstate-server:latest --stackpack-file ./my-stackpack.sts`