Extract a .net6/.netstandard2.1 core library

[buehler]

Some parts of KubeOps shall be extracted into a .netstandard2.0 "core library" such that other users of KubeObs may extend the core functionality. This is based on the discussion of #355.

[jefflill]

Hi guys, glad to see you're still at it! I've been using your old 6.2.4 release but am looking forward to upgrading this week.

I see that this issue is still open and I wanted to mention that I implemented a pretty easy workaround that your users could try in them meantime if these changes require too much surgery. Perhaps you could address this issue with some documentation/guidance with the pattern described below (and then close this issue):

I just created two library projects to hold my resources, one called ResourceDefinitions and the other called Resources, where:

• My ResourceDefinitions library project is .NET 6 and references the KubeOps packages and defines the CRDs via your attributes, etc.
• My Resources library project is .NETStandard and it just references (shares) the resource source files from the ResourceDefinitions project.
• The ResourceDefinitions project defines the KUBEOPS variable but the Resources project does not. I use conditional compilation to comment out the KubeOps attributes, etc. so the Resources project can build as .NETStandard and without referencing the KubeOps package.
• My operators reference the ResourceDefinitions library and your CRD generators work just fine against that.
• My client (non-operator apps) reference the .NETStandard Resources project and these resources can be managed just fine with KubernetesClient.

Here's an example of what one of my resource files look like:

using System;
using System.Collections.Generic;
using System.Text;

using k8s;
using k8s.Models;

#if KUBEOPS
using DotnetKubernetesClient.Entities;
using KubeOps.Operator.Entities;
using KubeOps.Operator.Entities.Annotations;
#endif

#if KUBEOPS
namespace Neon.Kube.ResourceDefinitions
#else
namespace Neon.Kube.Resources
#endif
{
#if KUBEOPS
    [KubernetesEntityShortNames]
    [EntityScope(EntityScope.Cluster)]
    [Description("Describes a neonKUBE cluster upstream container registry.")]
#endif
    public class V1ContainerRegistry : CustomKubernetesEntity<V1ContainerRegistry.V1ContainerRegistrySpec>
    {
        public V1ContainerRegistry()
        {
        }

        public class V1ContainerRegistrySpec
        {
            private const string prefixRegex = @"^(\*\.)?([a-zA-Z0-9-_]+\.)*([a-zA-Z0-9-_]+)(/[a-zA-Z0-9-\._~\[\]@\!&'\(\)\*+,;%=\$]+)*$";

#if KUBEOPS
            [Required]
            [Pattern(@"^(\*.)?" + prefixRegex)]
#endif
            public string Prefix { get; set; } = null;

            public int SearchOrder { get; set; } = -1;

            public bool Insecure { get; set; } = false;

            public bool Blocked { get; set; } = false;

#if KUBEOPS
            [Pattern(prefixRegex)]
#endif
            public string Location { get; set; } = null;

            public string Username { get; set; } = null;

            public string Password { get; set; } = null;
        }
    }
}