[FLINK-16517][Examples] Add an embedded input to enable a long-running WordCount example

[Ethanlm]

What is the purpose of the change

This PR modifies the current WordCount example to have a SourceFunction that randomly generates input data based on a set of sentences, so the WordCount job can run forever. The generation interval is configurable.

This will be the easiest way to start a long running flink job and can be useful for new users to start using flink quickly, or for developers to test flink easily.

Brief change log

• Added a new SourceFunction RandomSentenceSource that generates random data continuously until cancellation
• Modified the WordCount example to use the RandomSentenceSource as the input optionally

Verifying this change

This change can be verified as follows:

• Manually verified the change by running a 1 node cluster with 1 JobManagers and 1 TaskManagers, submitted the new WordCount jobs with --random option with different intervals and verified the jobs worked correctly.

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: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? JavaDocs

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Last check on commit 1df1ef1 (Wed Mar 18 02:24:22 UTC 2020)

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[tillrohrmann]

Thanks for creating this PR @Ethanlm. I was wondering whether the WindowJoin example is good enough as an easy example which can run infinitely. If this is the case, then we can close this PR and the respective JIRA issue.

[Ethanlm]

WindowJoin looks good.

I still feel like we need to change WordCount example so it can be a more streaming example. But it's up to you to decide whether doing it or not.

Thanks for the review. @tillrohrmann

[zentol]

Closing as Won't fix.