Allow using Cargo-as-a-library with gix's reqwest backend (#15653)

### What does this PR try to resolve?

Unconditionally enabling "blocking-http-transport-curl" made the `cargo`
library incompatible with crates that prefer reqwest. An example being
the `rustsec` crate with git support:

-
https://github.com/rustsec/rustsec/blob/rustsec/v0.30.2/rustsec/Cargo.toml#L45-L47
-
https://github.com/rustsec/rustsec/blob/rustsec/v0.30.2/rustsec/Cargo.toml#L34
-
https://github.com/EmbarkStudios/tame-index/blob/0.21.0/Cargo.toml#L14-L17

Having `cargo` and `rustsec` in the same dependency graph makes
`gix-transport` fail to compile.

```toml
[dependencies]
cargo = "0.88.0"
rustsec = "0.30.2"
```

```console
error[E0428]: the name `Impl` is defined multiple times
   --> $CARGO_HOME/registry/src/index.crates.io-1949cf8c6b5b557f/gix-transport-0.45.0/src/client/blocking_io/http/mod.rs:220:1
    |
217 | pub type Impl = curl::Curl;
    | --------------------------- previous definition of the type `Impl` here
...
220 | pub type Impl = reqwest::Remote;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Impl` redefined here
    |
    = note: `Impl` must be defined only once in the type namespace of this module

error: Cannot set both 'http-client-reqwest' and 'http-client-curl' features as they are mutually exclusive
  --> $CARGO_HOME/registry/src/index.crates.io-1949cf8c6b5b557f/gix-transport-0.45.0/src/client/blocking_io/http/mod.rs:26:1
   |
26 | compile_error!("Cannot set both 'http-client-reqwest' and 'http-client-curl' features as they are mutually exclusive");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

After this PR, dependency graphs that prefer reqwest can switch to
Cargo's http-transport-reqwest feature.

```toml
[dependencies]
cargo = { default-features = false, features = ["http-transport-reqwest"] }
rustsec = "0.30.2"
```

Cargo will continue to have a direct dependency on `curl`, but HTTP
operations performed through gix will use `reqwest`. This means both
curl's HTTP implementation and reqwest's HTTP implementation will be
linked. This is still much better than the only existing solution, which
is that you must pick versions of `cargo` and other dependency
(`rustsec`) which depend on semver-incompatible versions of gix, causing
2 entire versions of gix to be linked, in order to sidestep the mutually
exclusive features being enabled on the same version of gix. Gix version
numbers advance rapidly enough that this is often possible, but
sometimes (like right now) you would be unable to use the most recent
published release of `cargo`.

### How to test and review this PR?

- `cargo check --lib`
- `cargo check --lib --no-default-features --features
http-transport-reqwest`

Also tested by backporting this commit onto
<https://github.com/rust-lang/cargo/pull/15391>'s base commit (i.e. when
gix 0.70 was used) to ensure a conflict with rustsec's gix dependency,
and successfully building the following project.

```toml
[package]
name = "repro"
version = "0.0.0"
edition = "2024"
publish = false

[dependencies]
cargo = { path = "../cargo", default-features = false, features = ["http-transport-reqwest"] }
rustsec = "0.30.2"
```

Author: weihanglo