[IDEA] HTTP Sender should allow separate Proxy credentials from Target credentials

[ab-mg-23]

The current code for HTTP Sender applies credential auth scope to any host, any port, any realm. The credentials and authenticating mechanism for an authenticating forward proxy might not be the same as the target server.

Use case
Attempt to access an authenticating HTTP server via an authenticating forward HTTP proxy using an HTTP Sender destination.

Describe the solution you'd like
Allow authentication options for proxy separate from options for target HTTP server.

Describe alternatives you've considered
Write a JS destination that mirrors the Apache HttpComponents Client example for accessing an authenticated server via an authenticated forward proxy.