Easily mock a service

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Requirement

Currently we can not mock results for the service, we want to add the filter Mock[1] to the mesh pipeline to do it. One question here is whether we make it fixed like what we did, or add self-defined pre-filters/post-filters for the fixed pipeline(or as a new feature).

[1] https://github.com/megaease/easegress/blob/main/pkg/filter/mock/mock.go

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Closed because #54 addressed it.