Use reqwest w/ rusttls on Linux

[eoger]

Here's how our SSL stacks look like currently:

Platform	SSL stack
Android (phones)	viaduct -> GeckoView/NSS
Android (unit tests)	viaduct (broken since not initialized?)
iOS (phones + simulator)	nativetls > Secure Transport (OS)
Desktop (Win32)	nativetls > Schannel (OS)
Desktop (macOS)	nativetls > Secure Transport (OS)
Desktop (Linux)	nativetls > OpenSSL

In order to get rid of OpenSSL completely, I'd like use to move Android (Unit tests - Linux) from native-tls which pulls OpenSSL to rustls (SSL stack written in Rust).

My plan was to add to viaduct's Cargo.toml something like this:

[target.'cfg(not(any(target_os = "windows", target_os = "macos", target_os = "ios")))'.dependencies]
# reqwest w/ rustls feature
[target.'cfg(any(target_os = "windows", target_os = "macos", target_os = "ios"))'.dependencies]
# Regular reqwest

But because of rust-lang/cargo#2524 we can't do that 🙁

┆Issue is synchronized with this Jira Task
┆Sprint: Backlog

[rfk]

As a kind scoping question...how necessary is it for our unit tests to actually hit the network? Would an alternate approach here be to rework our unittests to use mocked-out network calls rather than depending on a live network connection? (I'm sure that would be a much bigger task, just want to understand the alternatives).

[eoger]

Turns out our unit tests actually don't hit the network at all, I discovered that they are compiled with Viaduct support which doesn't get initialized in unit tests and still pass :)

[rfk]

Alternately, we could consider making an iOS backend for viaduct, and then switch to using reqwest-with-rusttls on all desktop platforms (where it's only used for testing).

[eoger]

We did what we wanted to do here, and the reqwest/native-tls-vendored feature allowed us to remove OpenSSL from our libs/ directory.
Furthermore viaduct-reqwest must be imported explicitely to use the reqwest backend of viaduct now, so there's no confusion possible.