[V2] non-kube state serialization changes

[fgiorgetti]

Is your feature request related to a problem? Please describe.
Today, resources are serialized in directories under their respective state path.
For example: connectors/name.yaml

Describe the solution you'd like
Proposing a flat structure, with files named as:

<Kind>-<name>.yaml

Example:

Connector-my-connector.yaml
Listener-my-listener.yaml
Site-west.yaml