feat: allow specifying registry network in configuration

[limwa]

What

This PR allows users to specify a network to be used when creating a registry via a configuration file.

Example:

registries:
  create:
    ...
    network: k3d

Furthermore, a test was added to ensure that the "network" property is read from the config file. The documentation has been updated to let users know about this option.

This PR doesn't add the ability to specify a network via the --registry-create option.

I wasn't sure if this change required a new JSON schema version. Since this is not a breaking change, I decided to just add the property to the existing v1alpha5 schema.

Why

Fixes #1162.

[...]
Additional question: would it be possible to specify the --default-network parameter in the registries.create section of the k3d cluster create configuration?

Originally posted by @ar3s3ru in #1162

This is very useful for users using Podman. With this PR, they can create a bridge network beforehand and, in the config, use that network to create the registry (as recommended in the docs).

Implications

No breaking changes. If the network option is not specified, the "bridge" network remains the default.

[limwa]

@all-contributors please add @ar3s3ru for idea

[allcontributors]

@limwa

I've put up a pull request to add @ar3s3ru! 🎉