ext/zip: add ZipArchive::openString() method

Fixes to GH-14078:

* Rename ZipArchive::openBuffer() to ::openString().
* For consistency with ::open(), return int|bool, don't throw an
  exception on error. Provide error information via existing properties
  and accessors.
* Fix memory leak when ::openString() is called but ::close() is not
  called. Add test.
* Fix memory leak when a call to ::open() is followed by a call to
  ::openString(). Add test.
* Let libzip own the source, don't call zip_source_keep().
* Share buffer handling with ZipArchive::addFromString().

Elsewhere:

* If there is an error from zip_close() during a call to
  ZipArchive::open(), emit a warning but proceed to open the archive,
  don't return early. Add test.
* When buffers are saved by ZipArchive::addFromString(), release them
  in ZipArchive::close() and ::open(), don't accumulate buffers until
  the free_obj handler is called.
* Factor out buffer handling and reuse it in ZipArchive::openString()

Closes GH-21205.
Closes GH-14078.

Co-authored-by: Soner Sayakci <s.sayakci@shopware.com>
Co-authored-by: Ghaith Olabi <24876890+Gaitholabi@users.noreply.github.com>

UPGRADING

@@ -154,6 +154,9 @@ PHP 8.6 UPGRADE NOTES
     bound.
     RFC: https://wiki.php.net/rfc/clamp_v2
 
+- Zip:
+  . Added ZipArchive::openString() method.
+
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 7. New Classes and Interfaces
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