Add better support for external builders such as fabric-builder-k8s

[jt-nti]

What would you like to be added?

It would be great to improve the existing support for external builders to make it easy to use the fabric-builder-k8s builder with bevel-operator-fabric.

Why is this needed?

The fabric-builder-k8s builder is a Kubernetes-specific builder which would be a great fit for use with bevel-operator-fabric however, while the FabricPeerSpec does already include ExternalBuilder configuration, I think other aspects of bevel-operator-fabric makes assumptions about external builders which are not valid. For example, in terms of installing chaincode, the --k8s-builder flag, etc.