[feat]: add MySQL R2DBC query engine support

[zqr10159]

Summary

• add a built-in MySQL-compatible R2DBC query engine and wire it into collector strategy selection
• keep JDBC as an optional path when mysql-connector-j is provided from ext-lib, while covering MySQL, MariaDB, OceanBase, and TiDB SQL metrics
• add startup/runtime compatibility checks, e2e coverage, and documentation updates for native and JVM collector packaging

Verification

• ./mvnw -pl hertzbeat-startup -am -DskipITs -DskipE2E -Dsurefire.failIfNoSpecifiedTests=false -Dtest=ReactorNettyCompatibilityTest test
• ./mvnw -pl hertzbeat-collector/hertzbeat-collector-collector -am -DskipITs -DskipE2E -Dsurefire.failIfNoSpecifiedTests=false -Dtest="MysqlJdbcDriverAvailabilityTest,MysqlR2dbcJdbcQueryExecutorTest,CollectStrategyFactoryTest,NativeCollectorDefaultsTest" test

Notes

• branch was rebuilt from current apache/master so the PR only contains the MySQL R2DBC / compliance line of work