[FLINK-23495] [Connectors / Google Cloud PubSub] Make checkpoint optional for preview/staging mode

[brachi-wernick]

What is the purpose of the change

This PR add a new option for PubSub source called :withoutCheckpoint, to disable checkpoint verification.
This is useful for testing, and especially when using Ververica platform-> Sql editor (they disable checkpointing for preview cluster)

Brief change log

(for example:)

• new option is added to PubSubSource called :checkpointDisabled, to disable checkpoint verification.
• default value is false, meaning current behaviour will remain until explicit change to true.
• If value set to true, the method hasNoCheckpointingEnabled will not raise error anymore.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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: (yes / no / don't know) don't know

• The runtime per-record code paths (performance sensitive): don't know

• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: yes (checkpointing)

• The S3 file system connector: no

• Does this pull request introduce a new feature? yes, kind of,

• If yes, how is the feature documented? not documented (can you help me where should I document it)

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Automated Checks

Last check on commit aecf658 (Mon Jul 26 11:17:08 UTC 2021)

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[brachi-wernick]

Any chance I can get review on this? we talked on this direction in ticket FLINK-23495.
I would want also to add this to #16598