all: improve support for proxies

[bassosimone]

This epic is about improving support for proxies in all our applications. I discussed this topic with community members in a recent community meeting. This is a tentative list of improvements to add:

• we're working on TLS parroting and we should explore using parroting also for communicating with the backend (requires some more work than the work we have been doing so far, where we have mainly been focused on measuring);

• add support for proxy in the probe-cli;

• we should ensure we're proxying the communication with the web connectivity test helper if we're not doing it already;

• we stashed plans to automatically switch over to Psiphon when needed, but this is still one of the possible future steps.

We should group issues describing the above work into this epic.

[hellais]

Closing this in favour of: #2843