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GH-43141: [C++][Parquet] Replace use of int with int32_t in the internal Parquet encryption APIs (#43413)

### Rationale for this change

See #43141

### What changes are included in this PR?

* Changes uses of int to int32_t in the Encryptor and Decryptor APIs, except where interfacing with OpenSSL.
* Also change RandBytes to use size_t instead of int and check for overflow.
* Check the return code from OpenSSL's Rand_bytes in case there is a failure generating random bytes

### Are these changes tested?

Yes, this doesn't change behaviour and is covered by existing tests.

### Are there any user-facing changes?

No
* GitHub Issue: #43141

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