Add SSL client authentication options configuration for legacy RSA FTPS servers

[mamuleanu]

This PR adds a configuration hook that allows users to customize SslClientAuthenticationOptions before the TLS handshake, enabling support for Linux for legacy FTPS servers that only negotiate RSA key-exchange ciphers.

The problem:

On Linux, .NET’s SslStream does not offer RSA key-exchange cipher suites by default. As a result, FluentFTP cannot complete the TLS handshake on these servers unless it exposes SslClientAuthenticationOptions and allows custom CipherSuitesPolicy.

Microsoft documents this behavior here.

The commits added:

• Added ConfigureSslClientAuthenticationOptions property to FtpConfig that allows users to customize SslClientAuthenticationOptions before authentication
• Updated TLS handshake code to use SslClientAuthenticationOptions pattern in both synchronous and async methods
• Added example file demonstrating how to configure FluentFTP for legacy RSA-only servers

I've created a repository with instructions on how to reproduce the handshake error. The repository, including instructions on how to setup the environment, can be found here.

UPDATE:

As per PR feedback, the PR now adds an event handler that allows users to customize SslClientAuthenticationOptions before the TLS handshake, enabling support for legacy FTPS servers

Changes:

• Added FtpSslClientAuthenticationOptions event delegate and event args class - Infrastructure for the event
• Added ConfigureSslClientAuthenticationOptions event to BaseFtpClient - Event handler that allows customization of SSL options
• Added OnConfigureSslClientAuthenticationOptions helper method - Internal method to fire the event (following the same pattern as ValidateCertificate)
• Added example file - ConnectFTPSLegacyRSA.cs demonstrating how to configure FluentFTP for legacy RSA-only servers

[robinrodricks]

Thanks, good work. Event handlers should be added to the ftp client classes and not the config. Config is only for simple properties.

[mamuleanu]

@robinrodricks, thank you for the feedback. I have updated the implementation.