refactor(dns): remove unnecessary DNS client initialization

[ProBrian]

Summary

There are muliple duplcated dns initialization which is unnecessary, This PR aims to improve the logic of dns initialization which as less as possible.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix KAG-5059

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[ProBrian]

The dns init is done in kong/conf_loader/init.lua:

assert(require("kong.tools.dns")(conf))

From this point on, we can just use the initialized dns client which is already loaded.

[outsinre]

I wonder if we can update this https://github.com/Kong/kong-ee/blob/8157e937419695f142fad22e8d76fd98de074876/kong/globalpatches.lua#L536 to something like

client = require("kong.tools.dns")(kong and kong.configuration)

[ProBrian]

I wonder if we can update this https://github.com/Kong/kong-ee/blob/8157e937419695f142fad22e8d76fd98de074876/kong/globalpatches.lua#L536 to something like

client = require("kong.tools.dns")(kong and kong.configuration)

Seems kong.configuration is not yet init when global patch module first required, so kong and kong.configuration will not be applied, what do you think @chobits ?

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