Add RAT license audit config and compliance metadata for ASF release

[edespino]

This pull request introduces a full Apache RAT audit configuration to support ASF release readiness for Apache Cloudberry (Incubating) 2.0.0.

Key changes:

• Adds pom.xml configuration with custom apache-rat-plugin setup
• Disables default matchers and registers all known permissive licenses (BSD, MIT, ISC, PostgreSQL, Artistic, 0BSD, etc.)
• Includes matchers for GPL build files with Autoconf exceptions (e.g., config.guess, libtool.m4)
• Adds matcher coverage for legacy contributors (EMC, VMware, Broadcom, Pivotal, Greenplum), all treated under the Apache License 2.0
• Ensures licenseFamilyCategory values are unique to avoid matcher suppression
• Introduces README.apache.md to document licensing provenance and historical contributors
• Excludes the GPL-licensed file gpMgmt/bin/pythonSrc/ext/pylint-0.21.0.tar.gz from the audit
• Audit passes cleanly with 0 unknown licenses

To verify the audit locally:

mvn clean verify -Drat.consoleOutput=true

This will scan the full source tree using the defined license matchers and print a summary to the console. A detailed report will also be generated under target/rat.txt.

This ensures that the entire source tree can be reliably verified for ASF Category A compliance and prepares the project for an IPMC-reviewed source release vote.

Feedback welcome on matcher coverage, categorization, or formatting.