[C++][Parquet] WipeOutDecryptionKeys doesn't securely wipe out keys

[asfimport]

InternalFileDecryptor::WipeOutDecryptionKeys() merely call std::string::clear to dispose of the decryption key contents, but that method is not guaranteed to clear memory (it probably doesn't, actually).

We should probably devise a portable wrapper function for the various OS-specific memory clearing utilities.

Reporter: Antoine Pitrou / @pitrou

_{Note: This issue was originally created as ARROW-16202. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
cc @andersonm-ibm @itamarst

[asfimport]

Antoine Pitrou / @pitrou:
Actually we probably need a SecureString facility so that wiping happens every time a string copy is done.

[asfimport]

Antoine Pitrou / @pitrou:
Draft PR for a low-level helper here: #12890

[asfimport]

Krisztian Szucs / @kszucs:
@pitrou shall we postpone it to 9.0?

[asfimport]

Antoine Pitrou / @pitrou:
Yes, probably.

[asfimport]

Antoine Pitrou / @pitrou:
cc @ggershinsky

[pitrou]

@adamreeve For the record, you might be interested in this.