Remove unused columns when rebuilding projection on merge.

[KochetovNicolai]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

Remove unused columns when the projection is rebuilt during the merge. It reduces memory usage and creates fewer temporary parts.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[clickhouse-gh]

Workflow [PR], commit [928559d]

Summary: ❌

job_name	test_name	status	info	comment
Integration tests (amd_asan, flaky)		failure
	test_projection_rebuild_with_required_columns/test.py::test_projection_rebuild_uses_only_required_columns[2-3]	FAIL	cidb	IGNORED
	test_projection_rebuild_with_required_columns/test.py::test_projection_rebuild_uses_only_required_columns[3-3]	FAIL	cidb	IGNORED

[amosbird]

I've also optimized this part (see https://github.com/ClickHouse/ClickHouse/pull/93114/changes#diff-6c14341ca43dbbaef007e622cb9553ef92f4f32d3f0fd8c150ab7567678df545R984) by leveraging the fact that generated blocks always have monotonically increasing part offsets (both in merges and mutations). This allows us to pass only the starting offset to compute and construct the offset column efficiently.

[KochetovNicolai]

@amosbird thank you!
I would like to keep my PR as well, because it's backportable.

[alexey-milovidov]

I don't understand this test - what does it do?
As it fails every time, I will remove it.