feat(remote): add support for custom certificate validation callbacks

[Aaronontheweb]

Summary

This PR integrates the new CertificateValidationCallback feature from Akka.NET v1.5.55 into Akka.Remote.Hosting, allowing users to provide custom certificate validation logic for SSL/TLS connections.

Changes

• Updated Akka.NET to v1.5.55 - Required to access the new SSL/TLS features
• Added CustomValidator property to SslOptions - Allows users to provide a CertificateValidationCallback delegate
• Enhanced RemoteOptions.Build method - Now intelligently selects the appropriate DotNettySslSetup constructor:
  • 5-parameter constructor when CustomValidator is provided (v1.5.55+)
  • 4-parameter constructor when RequireMutualAuthentication or ValidateCertificateHostname is set
  • 2-parameter constructor for legacy scenarios (full backward compatibility)
• Added test coverage - New test verifies custom validators are properly configured
• Updated API approval tests - Reflects the new public API surface

Key Features

This enhancement enables advanced SSL/TLS scenarios including:

• Certificate pinning (whitelist known cert thumbprints)
• Subject/issuer matching (organizational CA validation)
• Custom business validation rules
• Advanced mTLS scenarios

Example Usage

services.AddAkka("RemoteSys", (builder, provider) =>
{
    builder.WithRemoting(new RemoteOptions
    {
        Port = 2552,
        EnableSsl = true,
        Ssl = new SslOptions
        {
            X509Certificate = myCert,
            CustomValidator = (cert, chain, peer, errors, log) =>
            {
                // Custom validation logic
                return IsValidCertificate(cert, peer);
            }
        }
    });
});

Backward Compatibility

✅ 100% backward compatible - All existing code continues to work unchanged. The new CustomValidator property is optional and the implementation maintains the existing constructor selection logic when it's not provided.

Testing

• All existing tests pass (16/16 in Akka.Remote.Hosting.Tests)
• New test specifically validates CustomValidator configuration
• API approval tests updated and passing
• No SSL/TLS functional testing added (we rely on Akka.Remote's comprehensive test coverage)

Notes

• As noted in Akka.NET v1.5.55 release notes, programmatic DotNettySslSetup configuration now correctly takes precedence over HOCON defaults, ensuring our settings are effective
• We use DotNettySslSetup for SSL configuration when a certificate object is provided, which guarantees the settings are applied

Resolves the need to support custom certificate validation scenarios as introduced in Akka.NET v1.5.55.

[Aaronontheweb]

LGTM