[FLINK-31486] Get the KeySelector with UserClassLoader

[jiangxin369]

What is the purpose of the change

Obtains the KeySelector with UserClassLoader instead of AppClassLoader to fix FLINK-31486.

Brief change log

• Obtains the KeySelector with UserClassLoader.

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)

Documentation

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

[jiangxin369]

@gaoyunhaii @lindong28 Could you help review this PR?

[lindong28]

Thanks for the PR. Can you add a unit test to cover the bug fixed by this PR?

[jiangxin369]

@lindong28 I wanted to add a unittest for the change, but I cannot reproduce the exception within a minicluster and then fix it. The reason is the user codes are also added to the java classpath of the unittest process and can also be loaded by AppClassLoader.

[lindong28]

Thanks for the PR! LGTM.