fix: api serializer for mapping updates required values

As CMDB user, I would like to modify required values on RPS plugin model So it fits expected behavior.

Bug description: When testing API , "PATCH" /api/plugins/rps/mapping/{id}/ If "Id" is provided, still fails if "source" and "target" are not provided on body. But those should not be required.

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Present behavior:

"PATCH" /api/plugins/rps/mapping/{id}/ { "source": "https://hey3.com", "target": "https://hey4.com", "operation": "none" }`

Expected behavior:

"PATCH" /api/plugins/rps/mapping/{id}/ { "operation": "none" }