Added IntoFuture for RequestBuilder

[jewlexx]

Implements the IntoFuture trait for RequestBuilder.

Only works for Rust versions greater than 1.64, hence I also added a feature flag for it so that it isn't enabled by default and won't break existing implementations on older Rust versions.

Closes #1632

[jewlexx]

#1632

[Suyashtnt]

Any updates on when this will be updated/merged?

[seanmonstar]

The hold up is that providing this impl would require all to upgrade their compiler version. While it's definitely useful, we've so far tried to only require compiler upgrades when it provides something very important (or at the least, the new MSRV we choose is 6+ months old).

One option is to use a build.rs file to conditionally add the impl if the compiler is new enough...

[jewlexx]

@seanmonstar That is why I added the feature flag. My hope was that end developers would only enable said flag if they had the required Rust version, and until rust-lang/rust#64796 is stabilised, that seemed like the best way, bar using a crate that enables that functionality such as https://github.com/dtolnay/rustversion

[jewlexx]

Another note on using rustversion to conditionally compile it is that it currently has an MSRV of 1.31 which is significantly lower than reqwest's current MSRV of 1.56, so it would only require an additional dependency.

[Sytten]

Given that this was 2 years ago, it probably ok to just use that as is now.