Add V2 internal SDMX endpoints and remote data source support

[keyurva]

• Defined internal V2 SDMX data and availability RPCs for inter-instance query federation.
• Implemented server handlers for internal SDMX requests.
• Added SDMX query delegation to the remote client and remote data source.

Manually tested by deploying to dev, and running mixer in dcp mode with the dev mixer as remote:

curl -X POST http://localhost:8081/v3/internal/sdmx/data \
  -H "Content-Type: application/json" \
  -d '{
    "constraints": {
      "variableMeasured": {
        "predicates": [{"value": "Count_Person"}]
      },
      "observationAbout": {
        "predicates": [{"value": "geoId/06"}]
      }
    }
  }' | jq .

[gemini-code-assist]

Code Review

This pull request introduces internal SDMX endpoints (V3InternalSdmxData and V3InternalSdmxAvailability) to support structured SDMX data and availability queries from remote mixers. It updates the protobuf definitions, implements the server handlers, adds remote client support, and integrates these into the remote data source. Unit tests are also added to verify that requests are correctly forwarded. There are no review comments, and I have no feedback to provide.

[rohitkumarbhagat]

Thanks Keyur!!