Add ClientSettingsPolicy CRD

[kate-osborn]

Proposed changes

Add ClientSettingsPolicy CRD.

Problem: Users need a way to configure the behavior between the downstream client and NGF. For example, be able to set the client max body size.

Solution: Add the ClientSettingsPolicy CRD, which provides a way to configure this behavior. Note: this PR contains the CRD only. A subsequent PR will add the implementation.

Testing: Verified CRD can be installed and tested the OpenAPI schema validation.

This PR also adds the category nginx-gateway-fabric to the NginxGateway CRD. This allows users to look up all the api-resources related to our controller.

kubectl api-resources --categories=nginx-gateway-fabric 
NAME                     SHORTNAMES   APIVERSION                   NAMESPACED   KIND
clientsettingspolicies   cspolicy     gateway.nginx.org/v1alpha1   true         ClientSettingsPolicy
nginxgateways                         gateway.nginx.org/v1alpha1   true         NginxGateway

Closes #1759

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

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