Fix StorageURL doing some of the query execution in single thread

[al13n321]

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 to CHANGELOG.md):

Fixed queries that read a Parquet file over HTTP (url()/URL()) executing in one thread instead of max_threads.

#58353 accidentally changed num_streams behavior in StorageURL. It's supposed to create min(num_streams, num_files) StorageURLSource-s, then resize the pipeline to num_streams, so that the rest of query execution is multithreaded even if there's only one file to read.

Before the fix:

EXPLAIN PIPELINE
SELECT     
    UserID,                                    
    toMinute(CAST(EventTime, 'DateTime')) AS m,
    SearchPhrase,          
    COUNT(*)
FROM url('https://clickhouse-public-datasets-us-west-2.s3.us-west-2.amazonaws.com/hits.parquet')
GROUP BY   
    UserID,
    m,
    SearchPhrase
ORDER BY COUNT(*) DESC
LIMIT 10
SETTINGS max_threads = 64

Query id: b576c185-98f8-4d8d-9bfb-e1079bdd3734

┌─explain────────────────────────────────┐
│ (Expression)                           │
│ ExpressionTransform                    │
│   (Limit)                              │
│   Limit                                │
│     (Sorting)                          │
│     MergingSortedTransform 64 → 1      │
│       MergeSortingTransform × 64       │
│         LimitsCheckingTransform × 64   │
│           PartialSortingTransform × 64 │
│             (Expression)               │
│             ExpressionTransform × 64   │
│               (Aggregating)            │
│               Resize 1 → 64            │
│                 AggregatingTransform   │
│                   (Expression)         │
│                   ExpressionTransform  │
│                     (ReadFromURL)      │
│                     URL 0 → 1          │
└────────────────────────────────────────┘

After the fix:

┌─explain──────────────────────────────────────┐
│ (Expression)                                 │
│ ExpressionTransform                          │
│   (Limit)                                    │
│   Limit                                      │
│     (Sorting)                                │
│     MergingSortedTransform 64 → 1            │
│       MergeSortingTransform × 64             │
│         LimitsCheckingTransform × 64         │
│           PartialSortingTransform × 64       │
│             (Expression)                     │
│             ExpressionTransform × 64         │
│               (Aggregating)                  │
│               Resize 64 → 64                 │
│                 AggregatingTransform × 64    │
│                   StrictResize 64 → 64       │
│                     (Expression)             │
│                     ExpressionTransform × 64 │
│                       (ReadFromURL)          │
│                       Resize 1 → 64          │
│                         URL 0 → 1            │
└──────────────────────────────────────────────┘

Old version before #58353:

┌─explain──────────────────────────────────────┐
│ (Expression)                                 │
│ ExpressionTransform                          │
│   (Limit)                                    │
│   Limit                                      │
│     (Sorting)                                │
│     MergingSortedTransform 64 → 1            │
│       MergeSortingTransform × 64             │
│         LimitsCheckingTransform × 64         │
│           PartialSortingTransform × 64       │
│             (Expression)                     │
│             ExpressionTransform × 64         │
│               (Aggregating)                  │
│               Resize 64 → 64                 │
│                 AggregatingTransform × 64    │
│                   StrictResize 64 → 64       │
│                     (Expression)             │
│                     ExpressionTransform × 64 │
│                       (ReadFromStorage)      │
│                       Resize 1 → 64          │
│                         URL 0 → 1            │
└──────────────────────────────────────────────┘

Benchmark: https://pastila.nl/?cafebabe/6ae75c0a672707eb39dee0de241cb628.html#GhbniwKxvMQSvZPyWQgL5w==

[robot-clickhouse]

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