chore: add new keepalive_pool field to ApisixUpstream CR

[slayer321]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches
• Documentation
• Refactor
• Chore
• CI/CD or Tests

What this PR does / why we need it:

Fixes #1883
adds the new field keepalive_pool for ApisixUpstream CR

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[slayer321]

I have made all the required changes but I was not able to test it as I'm still stuck on the CRD's generation. One of the ways I tried doing it below

controller-gen crd paths=./pkg/kube/apisix/apis/... output:crd:dir=./samples/deploy/crd/v1

It is generating a whole new files for me. Please do let me know how can I generated the CRD's for this project.

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