feat: Defines more structs and methods for a partitioned merge tree #13516

Job	Run time
Clippy	2m 9s
coverage	10m 43s
Check (windows-latest)	8m 55s
Build GreptimeDB binaries (ubuntu-20.04)	4m 12s
Check (ubuntu-20.04)	1m 44s
Rustfmt	24s
Toml Check	15s
Spell Check with Typos	17s
Sqlness Test with Kafka Wal (ubuntu-20.04)	2m 22s
Sqlness Test (ubuntu-20.04)	1m 42s
Compatibility Test	39s
	33m 22s

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