Fix GH-13071: Copying large files using mmap-able source streams may exhaust available memory and fail

Commit 5cbe5a538c disabled chunking for all writes to streams. However,
user stream

Fix GH-13071: Copying large files using mmap-able source streams may exhaust available memory and fail

Commit 5cbe5a538c disabled chunking for all writes to streams. However,
user streams have a callback where code is executed on data that is
subject to the memory limit. Therefore, when using large writes or
stream_copy_to_stream/copy the memory limit can easily be hit with large
enough data.

To solve this, we reintroduce chunking for userspace streams.
Users have control over the chunk size, which is neat because
they can improve the performance by setting the chunk size if
that turns out to be a bottleneck.

In an ideal world, we add an option so we can "ask" the stream whether
it "prefers" chunked writes, similar to how we have
php_stream_mmap_supported & friends. However, that cannot be done on
stable branches.

Closes GH-13136.

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