This series prepares us for working on #11567 - allow adding a GSI to a pre-existing table. This will require changing the implementation of GSIs in Alternator to not use real columns in the schema for the materialized view, and instead of a computed column - a function which extracts the desired member from the `:attrs` map and de-serializes it.
This series does not contain the GSI re-implementation itself. Rather it contains a few small cleanups and mostly - new regression tests that cover this area, of adding and removing a GSI, and **using** a GSI, in more details than the tests we already had. I developed most of these tests while working on **buggy** fixes for #11567; The bugs in those implementations were exposed by the tests added here - they exposed bugs both in the new feature of adding or removing a GSI, and also regressions to the ordinary operation of GSI. So these tests should be helpful for whoever ends up fixing #11567, be it me based on my buggy implementation (which is _not_ included in this patch series), or someone else.
No backports needed - this is part of a new feature, which we don't usually backport.
Closesscylladb/scylladb#20383
* github.com:scylladb/scylladb:
test/alternator: more extensive tests for GSI with two new key attributes
test/alternator: test invalid key types for GSI
test/alternator: test combination of LSI and GSI
test/alternator: expand another test to use different write operations
test/alternator: test GSIs with different key types
alternator: better error message in some cases of key type mismatch
test/alternator: test for more elaborate GSI updates
test/alternator: strengthen tests for empty attribute values
test/alternator: fix typo in test_batch.py
test/alternator: more checks for GSI-key attribute validation
Alternator: drop unneeded "IS NOT NULL" clauses in MV of GSI/LSI
test/alternator: add more checks for adding/deleting a GSI
test/alternator: ensure table deletions in test_gsi.py
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