[FLINK-28027][connectors/async-sink] Implement slow start strategy fo…

[sthm]

What is the purpose of the change

Implement slow start strategy for AIMD to avoid overloading destination on initialization.

Brief change log

Adapt initialization of the AIMD strategy to set the maximum initial rate to a low number rather than starting with the theoretical maximum.

Verifying this change

Please make sure both new and modified tests in this PR follows the conventions defined in our code quality guide: https://flink.apache.org/contributing/code-style-and-quality-common.html#testing

This change is already covered by existing tests, such as testInitialRateIsSetByConstructor.

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 (during initialization of the sink)
• Anything that affects deployment or recovery: 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:

• ebcdf2e Azure: FAILURE

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[sthm]

This is superseded by FLINK-28487.