Add feature flag to control gix transport impl

[DCjanus]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

The crate gix provides two features, blocking-http-transport-reqwest and blocking-http-transport-curl, but they cannot be set simultaneously.

Most of the time, this requirement is easy to meet, unless we depend on multiple crates that have chosen different features.

In deps.rs, we encountered this problem:

• deps.rs -> rustsec -> tame-index -> gix/blocking-http-transport-reqwest
• deps.rs -> crates-index -> gix/blocking-http-transport-curl

Perhaps we should expose two features: gix-curl and gix-reqwest, so that upper-level users can have more fine-grained control.

[Jake-Shadle]

It's not allowing me to push to your fork despite it saying so, please fix the issues if you can.