Use proxy_ssl_context when fowarding to both a HTTPS destination
    (which occurs when use_forwarding_for_https=True)
    and to a HTTP destination.

    Raise a warning if both a proxy_ssl_context and a ssl_context
    are provided when use_forwarding_for_https=True.

    Add tests to verify these cases and modify
    test_proxy_https_target_tls_error in test_proxy_poolmanager
    to account for this new behavior.

    Addresses urllib3#2577

Raise an exception if both a proxy_ssl_context and a ssl_context
are provided when use_forwarding_for_https=True, instead of
only raising a warning.

Modify tests to account for change.

Resolves urllib3#2577