[FLINK-27399][Connector/Pulsar] Change initial consuming position setting logic for better handle the checkpoint.

[syhily]

What is the purpose of the change

Brief change log

This task modifies the flink-connector-pulsar module, adds some new mechanisms to set the initial consuming position.

• Change StartCursor, add new useful methods, and deprecate the confused fromMessageTime() method.
• Change StopCursor, and add new useful methods.
• Introduce a new SplitAssigner for assigning the splits among the Pulsar readers. Make the partition assignment logic clear and testable.
• Change the start position seeking mechanism from Pulsar consumer API to Pulsar admin API. Don't reset the start position when the topic has a subscription.

Verifying this change

This change is already covered by existing tests, such as:

• PulsarSourceITCase
• PulsarSourceEnumeratorTest
• PulsarOrderedPartitionSplitReaderTest

We add new tests for new partition assign logic:

• NormalSplitAssignerTest
• SharedSplitAssignerTest

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): (yes)
• 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 introduces a new feature? (yes)
• If yes, how is the feature documented? (docs)

[flinkbot]

CI report:

• 7ea6b3c49eaf3f2aa32b63ca3846603520d126d8 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[MartijnVisser]

@PatrickRen Can you also have a look at this PR? I've understood that this PR should help resolve this blocker test stability FLINK-26721

[MartijnVisser]

@tisonkun I've understood from @wuchong that you might also want to help/have a look at this PR, therefore I've tagged you.

[tisonkun]

@MartijnVisser Thank you. I'll review the patch in this week. Actually I ever try to request myself as a reviewer but forget several times >_<

[MartijnVisser]

@syhily Can you resolve the latest review comments?

[syhily]

@tisonkun Finally, the ci turns green.