Remove an allocation during RSA key generation

[vcsjones]

When generating an RSA key, we are creating a CngProperty. Since CngProperty always created a defensive copy of data, this adds an internal constructor that accepts a ReadOnlySpan<byte> and copies it, with the added benefit that the source doesn't have to be a byte[] already.

So this effectively changes a new byte[4] to a stackalloc byte[4].

This also removes a full null checks that were not required.

[ghost]

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Issue Details

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When generating an RSA key, we are creating a CngProperty. Since CngProperty always created a defensive copy of data, this adds an internal constructor that accepts a ReadOnlySpan<byte> and copies it, with the added benefit that the source doesn't have to be a byte[] already.

So this effectively changes a new byte[4] to a stackalloc byte[4].

Author:	vcsjones
Assignees:	vcsjones
Labels:	area-System.Security
Milestone:	-