unification of is_deleted for RMT with LWD-based _row_exists

[UnamedRus]

Use case

Adjust _row_exists for better support in RMT:

1. Allow to insert _row_exists column in INSERT statement.
2. _row_exists = 0, effectively mark previous versions of row also "deleted" during SELECT FINAL query.
3. Do not remove _row_exists rows from RMT parts during merge (unconditionally, but in some cases they can be removed)

For second, it may require reorder of reading steps:

1. do not "filter out" _row_exists=0 row during Vertical FINAL step and keep it, if it's last version
2. drop it from final result set.

If, this will be implemented, it does mean that RMT will have LWD and ultra-LWD (without costly mutation), and reduce demand for is_deleted support.

[alexey-milovidov]

This will be difficult to understand by users.

[UnamedRus]

Interesting that Apache Doris have concept of visible for user DORIS_DELETE_SIGN (basically, what is_deleted currently does in ReplacingMergeTree)
https://doris.apache.org/docs/data-operate/delete/batch-delete-manual#usage-examples