Feat/Index Cover

[KKould]

What problem does this PR solve?

By checking whether the index being used overrides the columns output by the Scan, if so, the required values ​​are returned only from the index itself, avoiding table lookups.

fix: where is_null scanned extra data during indexing.

What is changed and how it works?

When performing an index_lookup in CoveredIndexImpl, the value portion of the key is parsed, deserialized, and assembled into a tuple for return.

The is_null key lookup is implemented by marking null values ​​before the value to distinguish whether a value is null.

Code changes

• Has Rust code change
• Has CI related scripts change

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Note for reviewer