[FLINK-37722] Eliminate redundant field initialization of PojoSerializer

[X-czh]

What is the purpose of the change

Perf: eliminate redundant field initialization of PojoSerializer.

Brief change log

Currently, PojoSerializer will first create a new POJO instance and initialize all the fields (in the createInstance() method), then deserialize and set fields. The field initialization within the createInstance() method is redundant, as all fields will be set later during deserialization anyway. We should eliminate it for better performance.

The TupleSerializer has already applied similar techique, see the usage of TupleSerializer#instantiateRaw.

Verifying this change

This change is already covered by existing tests.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

[flinkbot]

CI report:

• 0bef61571bcc4c02ab8c24a18aeab81d8fd6487f Azure: SUCCESS

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@JunRuiLee Could you help take a review?