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6e5d143a - GH-31603: [C++] Wrap Parquet encryption keys in SecureString (#46017)

GH-31603: [C++] Wrap Parquet encryption keys in SecureString (#46017) ### Rationale for this change Cryptographic keys must be kept private. Using the new `arrow::util::SecureString` is vital for storing secrets securely. ### What changes are included in this PR? Uses the `arrow::util::SecureString` introduced in #46626 for cryptographic keys throughout Parquet encryption. ### Are these changes tested? Unit tests. ### Are there any user-facing changes? APIs that hand over secrets to Arrow require the secret to be encapsulated in a `SecureString`. **This PR includes breaking changes to public APIs.** TODO: - provide instructions for migration Supersedes #12890. * GitHub Issue: #31603 Lead-authored-by: Enrico Minack <github@enrico.minack.dev> Co-authored-by: Antoine Pitrou <antoine@python.org> Co-authored-by: Antoine Pitrou <pitrou@free.fr> Signed-off-by: Antoine Pitrou <antoine@python.org>