HTTPCLIENT-2369: implemented HTTPS-proxy tunnelling (TLS-in-TLS)

[arturobernalg]

add opt-in support for HTTPS-proxy tunnelling (TLS-in-TLS) by introducing ProxyTlsConnectionOperator and ProxyTlsAsyncConnectionOperator and corresponding useHttpsProxyTunnelling() builder flags.
While standard proxy tunneling (CONNECT method) establishes a plain TCP connection to the proxy, many modern enterprise and corporate environments utilize HTTPS proxies for enhanced security and access control.

[ok2c]

@arturobernalg Does it actually work?

[arturobernalg]

@arturobernalg Does it actually work?

@olegk Tests for ProxyTlsConnectionOperator and ProxyTlsAsyncConnectionOperator pass, mocking double-TLS handshakes and socket binding for HTTPS proxy tunneling. Patch with code and tests attached for HTTPCLIENT-2369.

[ok2c]

@arturobernalg I mean does it really work with real proxy servers like Squid? Has this been tested? As I have already tried to explain it in the past, TLS over TLS is presently not possible due to i/o reactor design limitations in HttpCore. This makes me very, very doubtful.

[arturobernalg]

I didn't test it with a real case. Didn't have the way.

[garydgregory]

I didn't test it with a real case. Didn't have the way.

Can't something be set up with Docker and then driven with a Docker Maven plugin?