[FLINK-32695] [Tests] Migrated SimpleStringGenerator to Source V2 Implementation

[Poorvankbhatia]

What is the purpose of the change

This PR migrates CheckpointedStreamingProgram from the deprecated legacy SourceFunction API to the modern FLIP-27 Source API.

Brief change log

• Removed SimpleStringGenerator class that implemented the deprecated SourceFunction interface
• Configured a new SimpleStringGenerator based on FLIP-27 Source API. (Must be implemented inside the user JAR as user code, & Cannot depend on external source implementations)
• Updated imports to use FLIP-27 Source API classes

Verifying this change

Existing tests already cover this change, ClassLoaderITCase.

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): no
• 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? no

[Poorvankbhatia]

@afedulov PTAL Thanks.

[flinkbot]

CI report:

• 6d210dfdbab9c8213e19ea55589506a643a19e15 Azure: SUCCESS
• ff8c1e126a3f00487d72b83dc545b80c376dff86 UNKNOWN
• 0a5a1c88b3007041a53d5cc7fc2baaa64d2bbdf8 UNKNOWN

Bot commands

The @flinkbot bot supports the following commands:

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

[Poorvankbhatia]

@fapaul Rebased with master. PTAL

[Poorvankbhatia]

@fapaul Rebased with master. PTAL.