add plugin replace-responsebody

[zhangjiahaol]

Ⅰ. Describe what this PR did

Implement a plugin that can replace the response body according to the configuration

Ⅱ. Does this pull request fix one issue?

fixes #62

Ⅲ. Why don't you add test cases (unit test/integration test)?

No unit tests yet

Ⅳ. Describe how to verify it

If you deployed higress as a K8S cluster, you can use this yaml to test replace-responsebody.yaml

Ⅴ. Special notes for reviews

Check out README.md

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