Add SymmetricAlgorithm.SetKey(ROSpan)

[bartonjs]

This change adds SymmetricAlgorithm.SetKey to set the key of an instance based on a ReadOnlySpan.

The AesImplementation type takes advantage of it by moving the key to a FixedMemoryKeyBox. None of the other algorithms are being changed at this time, mostly because no one should be using them.

AesCng could also change to using FixedMemoryKeyBox, but it's a non-trivial change due to the externalization of state into the CngSymmetricAlgorithmCore struct; and the main intended purpose for the AesCng type is to open a persisted key (where the change wouldn't matter).

Fixes #111154.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[Copilot]

PR Overview

This pull request introduces a new SetKey(ReadOnlySpan key) API for symmetric algorithms. It refactors key management by using a FixedMemoryKeyBox in implementations such as AES and AppleCCCryptor, and it updates tests across AES, RC2, DES, TripleDES, and one-shot paths to verify the new SetKey functionality.

Reviewed Changes

File	Description
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/AES/AesContractTests.cs	Adds tests checking behavior when setting the key via property assignment and SetKey.
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/RC2/RC2CipherTests.cs	Introduces SetKey_Sanity tests to verify key exchange between instances.
src/libraries/Common/tests/System/Security/Cryptography/AlgorithmImplementations/DES/DESCipherTests.cs	Updates tests to use SetKey for ensuring consistency of key and IV handling.
src/libraries/System/Security/Cryptography/src/System/Security/Cryptography/AesImplementation.cs	Refactors key handling with FixedMemoryKeyBox support and overrides Key, KeySize and GenerateKey.
src/libraries/System/Security/Cryptography/src/System/Security/Cryptography/AppleCCCryptor.cs	Upgrades key storage to use FixedMemoryKeyBox and disposes the key in Dispose.
Other files (AesImplementation.OpenSsl.cs, AesImplementation.Windows.cs, ref/System.Security.Cryptography.cs, etc.)	Update API definitions and implementations to support SetKey overload with ReadOnlySpan.

Copilot reviewed 25 out of 25 changed files in this pull request and generated no comments.