[FLINK-39300][mysql-cdc] Reuse ObjectMapper as static constant to avoid repeated instantiation.

[eric666666]

What is the purpose of the change

This PR addresses FLINK-39300.

Jackson's ObjectMapper is expensive to instantiate (triggers reflection-based
module discovery and codec registry initialization), yet it is thread-safe once
configured. Two classes in the MySQL CDC connector were unnecessarily creating
new ObjectMapper instances on every method call:

• BinlogOffsetSerializer: both serialize() and deserialize() created new instances
• OnlineSchemaChangeUtils: both isOnLineSchemaChangeEvent() and parseOnLineSchemaRenameEvent() created new instances

This PR extracts ObjectMapper into a shared private static final constant in
each class to eliminate repeated instantiation overhead.

[eric666666]

/retest

[eric666666]

The test failure TransformE2eITCase#testWildcardWithMetadataColumnTransform is a flaky E2E test caused by waitUntilSpecificEvent timeout in the CI environment. It is unrelated to this PR's changes, which only refactor
ObjectMapper as a static constant in BinlogOffsetSerializer and OnlineSchemaChangeUtils. Please re-run the CI.
@yuxiqian Please help!