CASSSIDECAR-226 Adding endpoint for verifying files post data copy during live migration

[nvharikrishna]

CASSSIDECAR-226 Adding an endpoint for verifying files between source and destination post data copy.

This implementation uses a two-task approach (data copy + file verification) rather than inline digest verification during data copy (as originally proposed in CEP-40). This design choice is motivated by:

1. Performance Efficiency: The data copy task executes multiple iterations internally. Even with successThreshold=1.0, the task might require at least two internal iterations (iteration 0: download → DOWNLOAD_COMPLETE, iteration 1: verify threshold → SUCCESS). Inline digest verification would calculate digests twice per file (once in each iteration), doubling the I/O cost. With separate tasks, digests are calculated once after data stabilizes.
2. Code Simplicity: Separating digest verification from file copying provides clear separation of concerns, making each task easier to understand, test, and maintain.
3. Operational Flexibility: Users can run verification independently, repeat it if needed, or skip it for non-critical migrations. Inline verification would make this mandatory overhead.

Here are the endpoint details:

Sample files verification task submission request:

curl -X POST http://dest-host.example.com:9043/api/v1/live-migration/files-verification-tasks \
  -H "Content-Type: application/json" \
  -d '{
    "maxConcurrency": 10,
    "digestAlgorithm": "MD5"
  }'

It supports XXHash32 algorithm too and seed as additional input in the payload.

Sample response:

{
  "taskId": "b8e4f3d2-5c6b-5d9e-0f2g-3b4c5d6e7f8g",
  "statusUrl": "/api/v1/live-migration/files-verification-tasks/b8e4f3d2-5c6b-5d9e-0f2g-3b4c5d6e7f8g"
}

Fetching files verification task status

curl -X GET http://dest-host.example.com:9043/api/v1/live-migration/files-verification-tasks/b8e4f3d2-5c6b-5d9e-0f2g-3b4c5d6e7f8g

Sample response:

{
  "id": "b8e4f3d2-5c6b-5d9e-0f2g-3b4c5d6e7f8g ",
  "digestAlgorithm": "md5",
  "seed": null,
  "state": "COMPLETED",
  "source": "localhost1",
  "port": 9043,
  "filesNotFoundAtSource": 0,
  "filesNotFoundAtDestination": 0,
  "metadataMatched": 379,
  "metadataMismatches": 0,
  "digestMismatches": 0,
  "digestVerificationFailures": 0,
  "filesMatched": 323
}

Also made additional changes to ensure that either data copy task or file verification task can be executed at any point of time.