Implement ExportPkcs12

[vcsjones]

This implements ExportPkcs12 on X509Certificate and X509Certificate2Collection.

Most of this pull request is minor shuffling to allow pulling Pkcs12Info and the related ASN.1 codecs in to Common.

1. Non-Windows always exports things in terms of PbeParameters. Pkcs12ExportPbeParameters is mapped to the windows-equivalent.
2. Windows, when using Pkcs12ExportPbeParameters, will rely on the Win32 API, if it can.
   • Pkcs12TripleDesSha1 is always exported by PFXExportCertStoreEx.
   • Pbes2Aes256Sha256 is exported by PFXExportCertStoreEx with PKCS12_PBES2_EXPORT_PARAMS, if it is available (Windows 10, 1709+).
   • If PKCS12_PBES2_EXPORT_PARAMS is not available on the platform, it is exported as Pkcs12TripleDesSha1 and re-created with Windows-equivalent parameters.
3. Windows, when using PbeParamters, will export via PFXExportCertStoreEx, preferring PBES2 if it is available. It is then re-encoded with the specified PbeParameters.

Closes #80314

PR Overview

This pull request implements the ExportPkcs12 functionality for X509Certificate and X509Certificate2Collection and refactors several ASN.1 codec types and interop structures to share common code. Key changes include:

• Adding a new P/Invoke signature for PFXExportCertStoreEx and a corresponding struct for PKCS12_PBES2_EXPORT_PARAMS.
• Enhancing the PkcsHelpers with new helper methods for BER decoding and attribute normalization.
• Updating the visibility and namespace for types such as Pkcs12IntegrityMode and several ASN.1 types from the Pkcs to the Pkcs7 namespace.
• Introducing a new flag (PKCS12_EXPORT_PBES2_PARAMS) in the PFXExportFlags enum.

Changes

File	Description
src/libraries/Common/src/Interop/Windows/Crypt32/Interop.PFXExportCertStoreEx.cs	Added new P/Invoke signature for certificate export.
src/libraries/Common/src/Interop/Windows/Crypt32/Interop.PKCS12_PBES2_EXPORT_PARAMS.cs	Introduced new struct for PBES2 export parameters.
src/libraries/Common/src/Internal/Cryptography/PkcsHelpers.cs	Added new BER decoding helper and attribute set normalization method.
src/libraries/Common/src/System/Security/Cryptography/Pkcs/Pkcs12IntegrityMode.cs	Adjusted visibility based on build configuration.
src/libraries/Common/src/System/Security/Cryptography/Pkcs/Pkcs12Info.cs	Modified access level and removed unused using statement.
src/libraries/Common/src/System/Security/Cryptography/Asn1/Pkcs7/*.xml.cs	Updated various ASN.1 types to use the Pkcs7 namespace.
src/libraries/Common/src/Interop/Windows/Crypt32/Interop.PFXExportFlags.cs	Added export flag for PBES2 parameters.

Copilot reviewed 61 out of 61 changed files in this pull request and generated no comments.

Tip: If you use Visual Studio Code, you can request a review from Copilot before you push from the "Source Control" tab. Learn more

[bartonjs]

Couple of nits, but overall LGTM