Add public API definitions for surfacing data age

[jowlyzhang]

This PR adds the definition for the public APIs for surfacing data write time info. It only contains minimum implementation. The implementations will be in follow ups. I need to sync with customers if these public APIs meet their requirements and are easy to use. And make modifications accordingly before proceeding with implementations.

• struct DataCollectionUnixWriteTimeInfo is a struct for the unix write time info for a collection of data
• DB::GetPropertiesOfTablesForLevels returns table properties collection per level
• GetDataCollectionUnixWriteTimeInfoForFile returns the data write time info for a file.
• GetDataCollectionUnixWriteTimeInfoForLevels returns the data write time info for levels.
• The user property names for recording write time stats in the user collected properties are defined.

Follow ups:

• Implement collecting the write time related user table properties
• Use the data write time info recorded in the table properties to implement these APIs

Test plan: No functional change, also follow ups should have tests covering the minimum implementation added in this PR.

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