feat: Move simpleschema to its own module

[barney-s]

This PR moves the simple-schema package to its own Go module at the top-level of the repository. This is done to make the package more reusable and independent of the main kro module.

The following changes were made:

• Moved the simple-schema package to the top-level of the repository.
• Created a new go.mod file for the simple-schema module.
• Updated the import paths in the main module to use the new module.
• Updated the main go.mod file to use a replace directive for the new module.
• Moved the crd utils into simple-schema
• Add a new cli for generating CRD from simple schema file.

Fixes: #720

[k8s-ci-robot]

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[jakobmoellerdev]

I understand that you want to separate out development of simpleschema from kro mainline, but my question is if this really solves the problem of independance? if we think that development on KRO is too slow, we should fix that.

I think if we introduce a separate module that wont change our review practices so im afraid it wont achieve the desired goal. Also happy to hear other thoughts though and maybe I am missing things.

On a different note: We will have to start running separate releases for this and document it as well, bit unsure if we want to maintain overhead

On a final note, we should not merge this without a two step approach because of the replace statement. Currently the statement would prohibit people from calling "go get" and one wouldnt be able to use the "pkg" packages as imports anymore.

[k8s-ci-robot]

PR needs rebase.

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[a-hilaly]

Thank barni - i do have few thoughts:

1/ I think simpleschema is fundamentally about OpenAPI specs rather than CRDs. That's why I do not agree moving graph/crd into the simpleschema package.

2/ the new simpleschema/go.mod pulls in heavy Kubernetes dependencies (k8s.io/apiextensions-apiserver and k8s.io/apimachinery). If the goal is to make it "more reusable and independent" adding these dependencies works against that.

3/ If we truly want simpleschema to be independnt, it probably belongs in its own repository (like github.com/opensimpleschema) but its probably too early to commit to that kind of distributed development workflow.

[a-hilaly]

the CLI addition was unexpected and I'm not clear on why we need one now.