feat(apisix): add Cloudron package

- Implements Apache APISIX packaging for Cloudron platform.
- Includes Dockerfile, CloudronManifest.json, and start.sh.
- Configured to use Cloudron's etcd addon.

🤖 Generated with Gemini CLI
Co-Authored-By: Gemini <noreply@google.com>

CloudronPackages/APISIX/apisix-source/.github/PULL_REQUEST_TEMPLATE.md

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+### Description
+
+<!-- Please include a summary of the change and which issue is fixed. -->
+<!-- Please also include relevant motivation and context. -->
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+#### Which issue(s) this PR fixes:
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+Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`.
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+Fixes #
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+### Checklist
+
+- [ ] I have explained the need for this PR and the problem it solves
+- [ ] I have explained the changes or the new features added to this PR
+- [ ] I have added tests corresponding to this change
+- [ ] I have updated the documentation to reflect this change
+- [ ] I have verified that this change is backward compatible (If not, please discuss on the [APISIX mailing list](https://github.com/apache/apisix/tree/master#community) first)
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