[FLINK-25284][Table SQL / API] Add proportion of null values to generate with datagen

[snuyanzin]

What is the purpose of the change

The PR allows to specify the proportion of null values to generate with DataGen per field, like

CREATE TABLE Orders (
    order_number STRING,
    price        DECIMAL(32,2),
    buyer        ROW<id INT, last_name STRING>,
    order_time   TIMESTAMP(3),
    my_map       MAP<INT,STRING>,
    my_arrray    ARRAY<STRING>
) WITH (
   'connector' = 'datagen',
   'fields.order_number.null-rate' = '0.7',
   'fields.price.null-rate' = '1.0',
   'fields.order_time.null-rate' = '0.5',
   'fields.buyer.id.null-rate' = '0.5',
   'fields.buyer.null-rate' = '0.5',
   'fields.my_map.key.null-rate' = '0.5',
   'fields.my_map.null-rate' = '0.5',
   'fields.my_array.element.null-rate' = '0.1',
   'fields.my_array.null-rate' = '0.5'
);

Verifying this change

There have been added test cases to flink-table/flink-table-api-java-bridge/src/test/java/org/apache/flink/connector/datagen/table/types/DecimalDataRandomGeneratorTest.java

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? (yes)
• If yes, how is the feature documented? (docs)

[flinkbot]

CI report:

• ac8c2ebd78820927cd1f1f2a34728c7eeadbadcb Azure: SUCCESS

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Automated Checks

Last check on commit 47449f943151ff06ddc777edf50798901d3ea20b (Tue Dec 14 22:38:55 UTC 2021)

Warnings:

• Documentation files were touched, but no docs/content.zh/ files: Update Chinese documentation or file Jira ticket.
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[snuyanzin]

@slinkydeveloper sorry for the delay and thanks for your feedback, I rebased it since it was already moved to assertj

[snuyanzin]

@JingsongLi sorry for the poke Since you are one of the committers dealing with datagen, could you please have a look here, once you have time?

[snuyanzin]

Partially it was reviewed by @slinkydeveloper @MartijnVisser sorry for the poke. Could you please tell if you happen to know who can help with further steps here?

[snuyanzin]

Rebased to resolve conflicts after merging https://github.com/apache/flink/pull/19827

[slinkydeveloper]

I'm sorry @snuyanzin, unfortunately I won't have time for this

[snuyanzin]

@alpinegizmo sorry for the poke could you please have a look once you have time?

[snuyanzin]

Thanks @RyanSkraba for having a look