Initial commit: Apache Cloudberry (Incubating) release script

Introduce cloudberry-release.sh, a utility to stage Apache Cloudberry
(Incubating) release candidates. This tool automates version validation,
Git tag creation, source tarball assembly, GPG signing, checksum
generation, and artifact verification.

Key features:
- Validates version consistency across configure.ac, configure,
  gpversion.py, and pom.xml
- Enforces Git tag immutability with optional reuse if HEAD matches
- Verifies submodule initialization and clean working state
- Creates BUILD_NUMBER and includes it in the release tarball
- Archives submodules recursively into the source tarball
- Signs tarball using GPG (unless --skip-signing is passed)
- Generates SHA-512 checksum and verifies both signature and hash
- Moves final artifacts into a dedicated top-level artifacts/ directory
- Logs Git identity for traceability and validates user config

This version serves as the authoritative foundation for Apache
Cloudberry’s staged release automation workflow.

Author: edespino