NIFI-7835 Add authenticated SOCKS and HTTP proxy support for SFTP

[exceptionfactory]

Description of PR

NIFI-7835 Adds support for accessing SFTP servers through SOCKS 5 proxy servers that require authentication. Support for authenticated proxy server access also applies to HTTP proxy servers, resolving NIFI-7749.

NiFi SFTP components use SSHJ since version 1.10.0. SSHJ does not provide direct support for authenticated proxy server access, and the current NiFi implementation supports SOCKS and HTTP proxy access without authentication using a custom ProxySocketFactory. The java.net.Authenticator class does not support providing proxy username and password credentials for single instances of java.net.Proxy, instead requiring the use an Authenticator instance for the entire JVM using the Authenticator.setDefault() method. This approach is not suitable for NiFi components that may require different sets of credentials for different component instances.

This pull request integrates the BrokeredSocketFactory from the socketbroker library to support authenticated access to SOCKS and HTTP proxy servers from NiFi SFTP components.

Runtime configurations can be tested using proxy servers that implement standard SOCKS 5 and HTTP CONNECT protocols. The following open source proxy servers can be configured with authentication for testing:

• Dante SOCKS 5 Proxy
• Tinyproxy HTTP Proxy
• Squid HTTP Proxy

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[greyp9]

Really useful enhancement to the SFTP processors!

Tested ListSFTP and FetchSFTP using squid (installed) for HTTP proxy and dante (Docker image) for SOCKS proxy, using proxy user name / password configuration. Also checked ability of user/pass proxy to interoperate with SSH pubkey authentication; no problems there. Code looks great.

Congrats on your socketbroker distributable!

[gresockj]

Thanks for this contribution, @exceptionfactory! I don't have any problems with the code.
Given @greyp9 's runtime testing, I'll go ahead and merge.