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For table-function-based tables, the engine shown in system.tables could change over time. E.g. (edited):

:) CREATE TABLE a (a int) AS s3('http://some_addr/some_bucket/data.tsv', '', '', 'TSV');
:) select engine from system.tables where database = 'default' and name = 'a'
   ┌─engine───────┐
1. │ StorageProxy │
   └──────────────┘
:) select count() from a
:) select engine from system.tables where database = 'default' and name = 'a'
   ┌─engine─┐
1. │ S3     │
   └────────┘

This behavior (a) seems weird, and (b) made 02888_system_tables_with_inaccessible_table_function flaky. This PR makes the engine always be reported as 'StorageProxy'.

(Alternatively we could propagate the underlying engine name, probably by adding a new virtual function in ITableFunction and implementing it in all ~32 table functions. Or report the table function name instead of IStorage's name, that would be trivial. LMK if you think we should do that instead of 'StorageProxy'.)