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GH-40328: [C++][Parquet] Allow use of FileDecryptionProperties after the CryptoFactory is destroyed (#40329)

### Rationale for this change

See #40328

### What changes are included in this PR?

Changes `FileKeyUnwrapper` so it can hold a shared pointer to a `KeyToolkit` and remain usable after the `CryptoFactory` that created it is destroyed. Also changes `CryptoFactory` to store a shared pointer to a `KeyToolkit` instead of storing a `KeyToolkit` inline.

### Are these changes tested?

Yes, I've added a new unit test for this

### Are there any user-facing changes?

This changes functionality for users of the C++ API in a non-breaking way, allowing them to use the `FileDecryptionProperties` returned from a `CryptoFactory` without having to keep the `CryptoFactory` alive.
* GitHub Issue: #40328

Lead-authored-by: Adam Reeve <adreeve@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>