Add AddRegisteredId to SubjectAlternativeNameBuilder

[vcsjones]

Fixes #125127

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[Copilot]

Pull request overview

Adds support for adding a Registered ID (OID) GeneralName to SubjectAlternativeNameBuilder, enabling Subject Alternative Name (SAN) construction scenarios that require registeredID entries.

Changes:

• Added SubjectAlternativeNameBuilder.AddRegisteredId(string registeredId) API.
• Added unit tests covering argument validation, valid encoding, invalid OID handling, and mixed multi-value SANs.
• Updated the reference assembly to expose the new public API.

Show a summary per file

File	Description
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/SubjectAlternativeNameBuilder.cs	Adds the new AddRegisteredId API and routes it through GeneralNameAsn.RegisteredId.
src/libraries/System.Security.Cryptography/tests/X509Certificates/CertificateCreation/SubjectAltNameBuilderTests.cs	Adds coverage for registered ID encoding and invalid OID input behavior.
src/libraries/System.Security.Cryptography/ref/System.Security.Cryptography.cs	Exposes AddRegisteredId(string) in the public ref surface.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 1