Projection bug fixes and refactoring.

[amosbird]

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Changelog category (leave one):

• Bug Fix (user-visible misbehaviour in official stable or prestable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Multiple small fixes for projections. See detailed description in pr.

Detailed description / Documentation draft:

1. disable projection when sampling is in use.
2. proper handling empty_result_for_aggregation_by_empty_set for projections.
3. fix incomplete implementation of in-memory part projection.
4. load temporary tables before system tables so that projections can work in system tables.
5. enhance projection query analysis to match more queries, such as virtual columns and group by with empty keys.
6. fix projection desc lost in CTAS clause.
7. add projections Array(String) to system.parts.
8. add more comments and refactor/remove duplicate code.

It's the base PR of #26286 and it addresses several review sugguestions there.