feat: Add support for SOCKS4 (#610)

[Jaltaire]

This adds support for SOCKS4 proxies, implementing #610.

[Jaltaire]

Added a simple example here to test that everything works as expected: https://github.com/Jaltaire/reqwest/blob/examples/examples/socks4.rs.

We can merge this too, if desired.

[Jaltaire]

Also added a bump of tokio-socks to 0.5.2 since 0.5.2 makes SOCKS4 implementation feature-complete: https://github.com/sticnarf/tokio-socks/releases/tag/v0.5.2.

[Jaltaire]

@seanmonstar Would you mind taking a look at this?

[seanmonstar]

Thanks for working on this! Looks good, I'll start CI :)

[Jaltaire]

My pleasure. :)

@seanmonstar, I just re-pushed with the cargo fmt fixes. Would you mind re-running the CI?

[Jaltaire]

error: package `tokio-util v0.7.12` cannot be built because it requires rustc 1.70 or newer, while the currently active rustc version is 1.63.0

This doesn't seem to be related to my changes from what I can tell... Running cargo msrv even without my changes determines the MSRV to be 1.70.0. Perhaps previous CI jobs have been successful if there's some transitive dependency that's not pinned to a particular version and has been updated since? (Admittedly, I have not yet delved much into cargo dependency conflict resolution, so maybe something else at play of which I'm unaware.)

@seanmonstar How would you like to handle this? Can we bump to rust-version = "1.70.0"?

[seanmonstar]

I'll take care of it in #2412

[Jaltaire]

@seanmonstar Just rebased off your change. Can you kick off CI again?

[Jaltaire]

@seanmonstar, many thanks for helping get this through today.
Can you cut a new release version? I'm eager to use this change once you do. :)