Use ValueAsnReader in more places where it is easy to do so

[vcsjones]

We could drop in ValueAsnReader in a few places where AsnReader was being used, so let's get rid of those allocations.

I am trying to spanify the XML / XSLT loader next.

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[Copilot]

Pull request overview

Reduces allocations in ASN.1 parsing across the crypto stack by switching several straightforward AsnReader usages to the stack-only ValueAsnReader, aligning with existing “spanified” parsing patterns used elsewhere in System.Security.Cryptography.

Changes:

• Replaced AsnReader with ValueAsnReader in multiple X.509 and PKCS parsing paths where the reader does not need to escape.
• Updated integer reads to use ValueAsnReader.ReadIntegerBytes() span-returning API (removing .Span accessors).
• Updated IV/parameter parsing in PKCS managed PAL to use span-based input.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/X509EnhancedKeyUsageExtension.cs	Uses ValueAsnReader for EKU sequence decoding to avoid reader object allocation.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/ManagedCertificateFinder.cs	Uses ValueAsnReader for template name ASN.1 string decoding.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/EccKeyFormatHelper.cs	Uses ValueAsnReader for ECC specified-curve parameter decoding; updates integer byte reads to spans.
src/libraries/System.Security.Cryptography.Pkcs/src/System/Security/Cryptography/Pkcs/CmsSignature.cs	Uses ValueAsnReader for DER DSA signature parsing in DsaDerToIeee.
src/libraries/System.Security.Cryptography.Pkcs/src/Internal/Cryptography/Pal/AnyOS/ManagedPal.cs	Uses ValueAsnReader for IV octet-string parsing from algorithm parameters.
src/libraries/Common/src/System/Security/Cryptography/RSAAppleCrypto.macOS.cs	Uses ValueAsnReader for initial sequence/tag inspection during legacy key export.
src/libraries/Common/src/System/Security/Cryptography/RSAAppleCrypto.cs	Uses ValueAsnReader for initial sequence/tag inspection during key export.

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