[FLINK-36120][Runtime] Internal APIs for declaring the record processing and variables

[Zakelly]

What is the purpose of the change

The FLIP-455 introduces a set of internal APIs for declaring process. This PR implements the first part - declaring process blocks and variables.

Brief change log

Please check each commit.

• Internal APIs for declaring elements and timers processing
• Internal APIs for declaring variables that share across callbacks with same context
• Chain-style declaring
• An basic operator and function for those APIs.

Verifying this change

This change added tests AsyncKeyedProcessOperatorTest.

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

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): yes
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• f6b3f57 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build