Add macos-system-configuration feature flag

[littledivy]

The system-configuration crate was added in #1955 to support macOS system proxy settings. However, it forces users to link to the SystemConfiguration framework.

This PR makes macOS system proxy a default feature in reqwest. Users can disable the feature and avoid the dyld startup cost of loading the framework.

[seanmonstar]

I'm curious, does v0.11.25 (which includes #2130) fix it so that this isn't needed?

If it's still needed, we can include this in the 0.12 release that is coming up soon. But wanted to check.

[tnull]

I'm curious, does v0.11.25 (which includes #2130) fix it so that this isn't needed?

If it's still needed, we can include this in the 0.12 release that is coming up soon. But wanted to check.

@seanmonstar Yes, please include this fix. The inclusion of system-configuration let's our builds break left and right ever since reqwest shipped it. We now manually patch each release and probably will soon automate it. Moreover, since reqwest 0.11.25 shipped with #2130, MSRV is broken on macOS, see #2169.

Btw, given that MSRV keeps breaking, I'd highly suggest to re-add the Windows and macOS MSRV checks you had me drop in #1993.

[seanmonstar]

Merged this into 0.12-dev in #2185