refactor: new retention policy ByNumOfSnapshotsToKeep

[dantengsky]

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Summary

Introduce new table option data_retention_num_snapshots_to_keep, which allows users to specify the number of snapshots to retain for a table:

create  table t ... data_retention_num_snapshots_to_keep = N;

-- or
alter table t set options(data_retention_num_snapshots_to_keep = N);

this feature is designed to work with the enable_auto_vacuum setting, providing granular control over snapshot retention policies on a per-table basis

Example:

CREATE OR REPLACE TABLE t(c INT);

-- Set the table to retain only the most recent snapshot
ALTER TABLE t SET OPTIONS(data_retention_num_snapshots_to_keep = 1);

-- Enable automatic vacuum to trigger snapshot cleanup
SET enable_auto_vacuum = 1;

-- After each of these operations, only one snapshot will be kept
INSERT INTO t VALUES(1);
INSERT INTO t VALUES(2);
INSERT INTO t VALUES(3);

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[SkyFan2002]

LGTM

[BohuTANG]

Based on the good PR summary, the doc generated: databendlabs/databend-docs#1972 cc @soyeric128