feat: add multi-version Flink module structure (1.18/1.19/1.20)

[fightBoxing]

Summary

Convert the project from single-module to multi-module structure to support multiple Flink versions.

Changes

• Root pom.xml converted from single jar to multi-module parent pom with dependencyManagement and pluginManagement
• Added lance-flink-1.18, lance-flink-1.19, lance-flink-1.20 submodules
• Each submodule uses build-helper-maven-plugin to share source code from src/
• Added spotless, enforcer, and build-helper plugins
• Removed lance-flink-1.17 (EOL since 2024-10)

Verification

All 3 Flink versions compile successfully:

[INFO] Lance Flink Connector .............................. SUCCESS
[INFO] Lance Flink Connector - Flink 1.18 ................. SUCCESS
[INFO] Lance Flink Connector - Flink 1.19 ................. SUCCESS
[INFO] Lance Flink Connector - Flink 1.20 ................. SUCCESS