Describe the proposal

Refactor the Gravitino Flink connector to use a versioned architecture baseline and deliver Flink 1.18 support first.

This issue covers:

• introducing flink-common
• introducing flink-1.18
• introducing flink-runtime-1.18
• moving shared connector logic into flink-common
• adding version-specific Flink 1.18 catalog and factory entry classes
• adding Flink 1.18 version-specific integration test entries
• updating runtime packaging and CI for the Flink 1.18 lane

This issue is the first implementation stage under parent issue #9710.

Why is this needed

The current single-module Flink connector layout is not a good foundation for maintained multi-version support. A Flink 1.18 baseline is the first step to establish the new versioned architecture.

Additional context

Parent issue: #9710