[FLINK-18616][table sql/api]Add SHOW CURRENT DDLs

[fsk119]

What is the purpose of the change

Add show current ddls.

Brief change log

• Modify parser in flink and hive to parse SHOW CURRENT DDLs.
• Add Operation of SHOW CURRENT DDLs and convert sqlNode to corresponding Operation.
• Modify TableEnv to adapt to new Operation and return result according to input Operation type.

Verifying this change

This change added tests and can be verified as follows:

• Added test that parser can parse SHOW CURRENT DDLs with flink parser, hive parser and sql command parser.
• Added test that SHOW CURRENT DDLs can return correct result with all kinds of TableEnvironment(TableEnvironmentImpl, StreamTableEnv and BatchTableEnv) and sql client.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

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Automated Checks

Last check on commit c3a5fb2 (Mon Jul 20 07:03:27 UTC 2020)

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Review Progress

• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• a6485e7 Azure: SUCCESS

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[JingsongLi]

Thanks @fsk119 for your contribution, left some comments.

[JingsongLi]

Looks good to me.