refactor: clarify the meaning of table in TableCatalog and TableFragments

[xxchan]

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What's changed and what's your intention?

What does "table" mean is quite confusing

• Added some comments in proto to clarify. Didn't modify proto in case of incompatibility.
• Renamed some type and field that only exist in memory.

Checklist

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[xxchan]

• #19510 👈 (View in Graphite)
• #19509 : 1 other dependent PR (#18935 )
• main

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