[FLINK-35822] Introduce DESCRIBE FUNCTION

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What is the purpose of the change

This PR introduces DESCRIBE FUNCTION syntax, allowing users to describe the metadata of a given function. The design for this is covered under FLIP-465: https://cwiki.apache.org/confluence/display/FLINK/FLIP-465%3A+Introduce+DESCRIBE+FUNCTION

Brief change log

• Add Parser support for the next syntax
• Add FunctionDefinition as a new optional parameter for ContextResolvedFunction
• The bulk of the implementation logic is in DescribeFunctionOperation

Verifying this change

This change added tests and can be verified as follows:

• Added end to end tests in flink-table/flink-sql-client/src/test/resources/sql/function.q
• Added unit tests in FlinkSqlParserImplTest and SqlOtherOperationConverterTest

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 introduce a new feature? yes
• If yes, how is the feature documented? docs / JavaDocs

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CI report:

• 5e4992105cd9ba4917c03a05af157548ad4396cf UNKNOWN
• 86c0b0afad13fc1b6ec159e3274abd36eda3132b Azure: SUCCESS

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