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GH-41431: [C++][Parquet][Dataset] Fix repeated scan on encrypted dataset (#41550)

### Rationale for this change

When parquet dataset is reused to create multiple scanners, `FileMetaData` objects are cached to avoid parsing them again. However, these caused issues on encrypted files since internal file decryptors were no longer created by cached `FileMetaData` objects.

### What changes are included in this PR?

Expose file_decryptor from FileMetaData and set it properly.

### Are these changes tested?

Yes, modify the test to reproduce the issue and assure fixed.

### Are there any user-facing changes?

No.
* GitHub Issue: #41431

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Gang Wu <ustcwg@gmail.com>