Add pre-commit configuration to run kubeconform

[ellieayla]

Contribute a pre-commit hook specification, so https://pre-commit.com/ can discover how to run kubeconform.

Usage, in a project's .pre-commit-config.yaml

 - repo: https://github.com/yannh/kubeconform
   rev: v0.7.1  # pinned (future) version of kubeconform

   hooks:
   - id: kubeconform
     name: kubeconform run with defaults

   - id: kubeconform
     name: kubeconfirm run with 8 goroutines and extra schemas from crd catalog
     args:
     - -strict # disallow additional properties not in schema or duplicated keys

     # Look in both the default & contributed CRDs-catalog for schemas
     - -schema-location
     - default
     - -schema-location
     - 'https://raw.githubusercontent.com/datreeio/CRDs-catalog/main/{{.Group}}/{{.ResourceKind}}_{{.ResourceAPIVersion}}.json'

     # skip CRDs and Kustomization resources
     - -skip
     - CustomResourceDefinition,Kustomization

     # parallel
     - -n
     - "8"

$ pre-commit run --files default/whoami.yaml --verbose kubeconform

kubeconform run with defaults..............................................................Failed
- hook id: kubeconform
- duration: 0.36s
- exit code: 1

default/whoami.yaml - IngressRoute whoami failed validation: could not find schema for IngressRoute

kubeconfirm run with 8 goroutines and extra schemas from crd catalog.......................Failed
- hook id: kubeconform
- duration: 0.24s
- exit code: 1

default/whoami.yaml - IngressRoute whoami is invalid: problem validating schema. Check JSON formatting: jsonschema validation failed with 'https://raw.githubusercontent.com/datreeio/CRDs-catalog/main/traefik.io/ingressroute_v1alpha1.json#' - at '/spec': additional properties 'middlewares-spelling-mistake' not allowed

[ellieayla]

Just noticed that this idea was also proposed in #198 -- they're not identical changes. Probably only want to merge one & reject the other.

[lalten]

@yannh would be great to land this :)