[FLINK-29004]PyFlink supports specifying index-url when installing third-party dependencies

[jlon]

What is the purpose of the change

PyFlink supports specifying index-url when installing third-party dependencies。
When we use pyflink, if there is a third-party dependency, when using pip to install the package, it is retrieved and obtained from pypi.python.org by default, which is greatly affected by the network, slow download speed and easy to cause connection timeout. At this point, we think we can specify the index-url.

Brief change log

• Supports specifying index-url through the parameter python.requirements.index-url when we install third-party dependencies.

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:

• 3ad4107 Azure: FAILURE

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[Vancior]

Thanks a lot for the PR! Could you add a simple test to verify it, like what it does for other requirement configurations?
BTW, PR merging will available right after code freeze for release-1.16.

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