https://github.com/milvus-io/milvus/issues/35112
This pr would not affect milvus functionality by now.
It implments a Chunk memory layout that looks like
```
VariableColumn
|offset|offset|offset|
|data|data|data|
```
We maybe move offsets to the beginning and add null bitmaps later but
not in this PR.
And mmap test will also be added in another PR.
---------
Signed-off-by: sunby <sunbingyi1992@gmail.com>
