feat: add SecurityContext support for mTLS (mutual TLS)

[icd360sevofficial]

Summary

Add an optional securityContext parameter to ClientBase, ImapClient, and SmtpClient to enable mutual TLS (mTLS) / client certificate authentication.

This is needed for mail servers that require client certificates for authentication (e.g., HAProxy with verify required). Currently there is no way to pass a SecurityContext to the underlying SecureSocket.connect() call.

Changes

• ClientBase: new optional SecurityContext? securityContext field, passed to SecureSocket.connect() and SecureSocket.secure()
  • Also fixes a bug: upgradeToSslSocket() was not passing onBadCertificate to SecureSocket.secure()
• ImapClient: forward securityContext to ClientBase super constructor
• SmtpClient: forward securityContext to ClientBase super constructor

Usage

final context = SecurityContext()
  ..useCertificateChainBytes(utf8.encode(clientCertPem))
  ..usePrivateKeyBytes(utf8.encode(clientKeyPem));

final imapClient = ImapClient(
  isLogEnabled: true,
  onBadCertificate: (_) => true,
  securityContext: context,
);
await imapClient.connectToServer('mail.example.com', 993);

Backwards Compatibility

The parameter is optional and defaults to null. Existing code is completely unaffected — no breaking changes.

Motivation

We run a mail server with strict mTLS enforcement (HAProxy verify required). Without this change, we had to maintain a local fork of enough_mail just for these ~20 lines. This PR eliminates the need for that fork.

[robert-virkus]

Thank you very much for your contribution and sorry for the long processing time!