Hetzner DS does not configure private network interfaces

# Bug report

The Hetzner DataSource does not handle metadata server URLs of the form http://169.254.169.254/hetzner/v1/metadata/private-networks which are used to specify network interface configuration for interfaces attached to private networks.

Hetzner documentation for this is here: https://docs.hetzner.cloud/ and specifically the "Example: Private Networks" section shows an example of the information returned.

## Steps to reproduce the problem

On Heztner Cloud:
- Create one, or more, private networks
- Create a cloud instance with at least one interface attached to a private network

## Environment details
- Cloud-init version: all
- Operating System Distribution: all
- Cloud provider, platform or installer type: Hetzner

## cloud-init logs

Start at the Hetzner DataSource handling metadata server URLs and read the Hetzner documentation's “Example: Private Networks” section to understand the returned information. Confirm that an instance with private-network interfaces receives the corresponding network interface configuration, and add coverage for the private-networks metadata URL.

Written by the indexing model from the issue text.