CI: Add SHA256 checksum files for C++ client packages

[hongzhi-gao]

Description

Generate a .sha256 sidecar for each client-cpp zip artifact so downloads can be verified.
https://github.com/hongzhi-gao/iotdb/actions/runs/26022182886/job/76486318383

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This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods.
• added or updated version, license, or notice information
• added comments explaining the "why" and the intent of the code wherever would not be obvious
  for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold
  for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

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Key changed/added classes (or packages if there are too many classes) in this PR

[HTHou]

iotdb/distribution/pom.xml

Line 161 in 4ac0b7e

<artifactId>checksum-maven-plugin</artifactId>

There is a maven plugin that can create checksum easily.